Welcome to the National Center for Victims of Crime

We are the nation's leading resource and advocacy organization for crime victims and those who serve them. Please join us as we forge a national commitment to help victims of crime rebuild their lives.

DNA Answers

For technical assistance regarding forensic DNA and crime victims, email DNAanswers@ncvc.org.

Profiles in DNA

Norman Gahn

Assistant District Attorney for Milwaukee County, Wisconsin


An interview with Norman Gahn, Assistant District Attorney for Milwaukee County, Wisconsin.

What types of cases have you focused on in your work as Assistant District Attorney for Milwaukee County?

Gahn: Most of my work for the past 20 years -- both here in Milwaukee and as a special prosecutor for the state -- deals with homicide and sexual assault cases, especially cases that rely heavily on the various forensic science disciplines --particularly DNA. I did the first DNA case in Milwaukee County -- a serial murder case from the late 1980s and early 1990s when DNA was in its infancy. DNA evidence became the lynchpin for the case, which was a big one, and we secured a conviction. My boss wanted me involved because of my master's degree in forensic science and my previous 10 years of experience as a criminal investigator and military police officer for the army. 

What role has forensic DNA played in the cases you have prosecuted?  Why is DNA important to the work you do?

Gahn: All criminal cases are different, and we can use the available DNA technology in a variety of ways. Often, it's for identification -- which generally provides proof beyond a reasonable doubt -- the standard we have to meet for a jury to find guilt. DNA evidence has also been very useful in corroborating victim statements. It clearly backs up and supports what a victim says and gives an added level of credibility to their testimony. I have used it to help clear up and interpret a crime scene, to establish a "flight path" to give the jurors an idea of what happened at the scene. It is helpful in establishing the presence or absence of someone at a crime scene, and it"s been so very useful to impeach the statements of criminal defendants. Many times a criminal defendant will be arrested at the crime scene or shortly thereafter and will give a statement that changes dramatically once the results of DNA tests become available. Quite simply, it is probably the best thing we have going to help determine guilt or innocence.

Can you give us a few examples of important cases solved by DNA evidence?

Gahn: One the most satisfying cases we ever solved was from the early days of DNA, back in the 1980s. We had a number of sexual assaults of elderly women in their homes. These were vile, brutal assaults -- particularly heart-wrenching crimes. There was a suspect, but we didn't have enough evidence to arrest him. We did have semen from all the crime scenes, and various evidentiary items containing semen were sent to an independent DNA lab for testing. DNA testing showed that the DNA profiles from semen found on each of the victims matched the defendant. None of the victims could identify the assailant because they had been blindfolded and attacked at gunpoint. It gave us tremendous satisfaction to be able to go to trial on that case, convict the defendant, and send him to prison for the rest of his life. I will always remember that trial, and it was the DNA evidence that did it

In 2009, our office charged a serial murderer with seven counts of homicide for the deaths of seven women. These homicides spanned 21 years, with the first homicide in 1986. The cold case unit of the Milwaukee Police Department examined all unsolved homicides, and through DNA testing at the crime lab, was able to connect these seven cases. The same DNA profile developed from each of the homicides was eventually matched to the suspected serial killer. This very compelling, persuasive, powerful DNA helped us build the cases that caused this serial murderer to walk into the courtroom last year and plead guilty to all seven homicides. He was sent to prison for the rest of his life.  

How have recent advances in forensic DNA affected your ability to prosecute cases and your overall approach to these cases?

Gahn: On many levels, we have seen continued advances in the technology and the interpretation capabilities at the crime lab. We have advanced from the old ABO enzyme serology-type testing [blood type system] to the DNA RFLP testing [sometimes called DNA fingerprinting or profiling] to DQ Alpha and polymarker and into the current STR [short tandem repeat] technology. Each one along the way offered a little better technology and maybe allowed us to test an additional biological substance or required smaller amounts for testing. In the 1990s, we worked into the STRs that we have today, which have enabled us to test even smaller amounts -- little specs of a biological substance, as well as a greater range of biological substances. From these tests, the crime lab can develop profiles and establish more and more associations among individuals and crime scene items. The interpretation capabilities of the crime labs and the statistical analysis have gone from the old ABO of 1 in 100 to polymarker DQ Alpha of 1 in a 1000 to the RFLP of 1 in a million, and now the current STR technology allows the crime lab analyst to attribute the source to a biological substance. Although less compelling statistically, mitochondrial DNA  testing and Y chromosome testing provide alternative testing methods when insufficient amounts of DNA are present for STR testing. And, of course, the ever-growing DNA databanks provide more opportunities to make associations between crime scenes and associations between crime scenes and convicted offenders.

What are some innovative ways that you have used DNA in the courtroom?

Gahn: John Doe DNA warrants are an innovative way to continue the hunt for criminals who have left their DNA at crime scenes. John Doe warrants are issued against a DNA profile, in other words, against an unnamed individual who has that particular DNA profile. Once we have issued those warrants, the statute of limitations stops running, and we can preserve jurisdiction and bring those cases back into court whenever we match the evidence from the crime scene to a particular individual. It's also interesting that the DNA technology has ushered in a new wave of visual aids in the courtroom, generally via PowerPoint. To effectively explain the DNA evidence -- which can be quite complicated -- we need to use visual aids for judges at admissibility hearings and for juries during trials. We can enlarge crime scene photographs and put them into a PowerPoint program, for example. It’s been an innovative way to use DNA in the courtroom.

We've also used the DNA in our Chapter 980 cases, which are our sexual predator civil commitments. On many occasions, I would have a case that I couldn't prosecute because it was beyond the statute of limitations, but the case had powerful, compelling DNA matches linking the defendant to the crime. I remember one sexual assault case from the early 1990s. By the time we got the cold hit out of the databank, the statute of limitations had passed and we could not charge the case. 

Nevertheless, I asked the police go to the prison and interview the suspect. He claimed he had never met the victim and had never been in the area where the rape had taken place. But the DNA evidence clearly showed otherwise. So I packaged up all the police reports and the crime lab reports showing that the semen on the vaginal swab from the victim matched the suspect. I sent the packet to the parole board at the prison system and asked them to use it in future parole hearings or 980 commitment hearings. The prison authorities appreciated this additional information and kept it in his file, which traveled with him from prison to prison. Just last year, there was a 980 hearing, and the judge decided to commit this person rather than release him; the decision was based on the sexual history, including the evidence about the uncharged sexual assault case I provided. Although I could not charge this case because of the expiration of the statute of limitations, the compelling DNA match was instrumental in keeping this person in prison. 

How did you start issuing John Doe DNA warrants?  

Gahn: Back in 1998, we were going through our old, unsolved sexual assault cases in Milwaukee and getting them submitted to the crime lab for analysis. Although databanking was in its infancy, we started getting hits from the evidence we submitted on cases we had thought we would never solve. Many of the original hits were within the statute of limitations. But then we started getting hits on cases where the statute had expired, and we could not proceed. That's when we decided that shortly before the statute was about to expire (i.e., five years, eleven months after the crime), we would issue a John Doe DNA warrant and charge the individual's DNA profile. By doing that, we preserved jurisdiction of the case. The clock stopped, the statute was tolled, and we could continue seeking the criminal. So we would get hits out of the DNA databank years and years later after the statute of limitations expired and still could successfully prosecute people when these hits occurred. These warrants became another wonderful tool that DNA gave us to continue the prosecutions of old cases -- and the passage of time no longer stopped that.    

Has the law itself been changed in response to these advances?    

Gahn: John Doe warrants were the impetus for many states to change their statutes of limitation, as we have done in Wisconsin. Especially on first- and second-degree sexual assault, we now don't have to issue John Doe warrants anymore -- the legislature has significantly changed the law. That has been happening throughout the country, all driven by DNA technology, and there's now a widespread belief that we don't need the statute of limitations anymore for those cases where you have a DNA profile. We also have new admissibility statutes and evidence retention policies that have been legislatively mandated because of the power of DNA. And, of course, there are more DNA post-conviction motion statutes, which allow access to evidence for DNA testing for those convicted individuals who believe that such testing would exonerate them. Many changes have been made to the law -- all driven by this great gift of forensic DNA testing.  

What current trends in the application of forensic DNA do you consider promising, and why?

Gahn: The important trends have to do with making improvements to and expanding upon the technology we have available now, especially in the areas of databases and DNA databanking. Expanding convicted offender collection laws is a good place to start. Many states are now collecting arrestee DNA -- not just from convicted offenders, but also from those arrested for a certain set of crimes. As more arrestees are swabbed and their profiles put into the databank, more crimes will be solved. Additionally,  more and more states and labs are using familial DNA searching. For instance, if a crime scene DNA sample does not match anyone in the convicted offender databank, there is a chance that the DNA of a relative of the unknown suspect may be in the databank. A familial search may result in a list of candidates that might include a brother or other close relative. It allows for partial matches that may provide investigators with new leads to pursue in cold cases. Familial searching is on the horizon, and I believe it offers a great deal of potential and will be used more and more as we go forward.  

In your experience, what kinds of opposition do these approaches generate?

Gahn: Of course, there is some opposition on the basis of Fourth Amendment (search and seizure) concerns -- less for arrestees than for family searches -- for fear these searches might implicate innocent people. There certainly is some legitimacy to these concerns, and they should be raised in the public forum. But in my opinion, these searches are powerful investigative tools that are available to us. If we legitimately have profiles in our possession and if we can use them to further an investigation or find someone who should be taken off the streets, I think that wins out. 

How have these scientific and legal changes helped victims?  

Gahn: It would take me several days to answer that question. I would highlight what this technology has done in the courtroom: the John Doe warrants, the changes in the statute of limitations, that ability to reach back in time and solve these old, cold cases that have been long forgotten -- not only by the police departments but by the victims themselves who had been thinking that no one was concerned about them anymore. It's been so helpful to victims to know that we can keep looking for the people who committed crimes against them and hold them responsible. It also gives them a choice whether they want to proceed; they don't have to, but it is wonderful to know that someone is still looking at these cases and trying to find who is responsible for inflicting a terrible injury on them. 

Victims are helped because the DNA technology allows investigators to go into a crime scene and look for biological evidence that is left there. Before DNA, that evidence was there, but there was just no way for investigators and lab analysts to test it and find the kind of associations that we now can use to solve the crimes. At a sexual assault scene, besides semen and blood, there can also be sweat or saliva, additional biological substances that give the crime lab more evidence to test and make associations. Testing can be done on condoms, clothing, linens, fingernail scrapings, ligatures, and tape. Victims are sometimes bound in some fashion, and it's hard for perpetrators to handle those bindings with gloves. So they may handle these items with bare hands or put the bindings in their mouths and leave saliva on them. Also, if we catch the suspect soon after the crime, we can do penile swabbing or test his body or clothing for the victim's epithelial cells. None of this was thought of in the past. What technology offers today is a more expansive, meaningful, complete, and compelling analysis of the crime scene, which can only help victims. 

How do these advances affect victims in court?

Gahn: DNA gives victims a powerful voice and relieves them of enormous pressure. Cases used to be based almost entirely on the victim testimony, and that allowed the defense to challenge the victim's credibility. But now, after a victim testifies, she is followed by the crime scene analyst who can testify that the semen found on the vaginal swab matches the DNA of the guy sitting across the courtroom at the defendant's table. And we can go to trial on cases where the victim was unable to identify the accused -- or even if the victim picked the wrong person out of the lineup. Those things used to be death knells for our cases, but now we can go forward on the basis of DNA evidence. Also, we've had so many cases involving children who are non-verbal or victims with communication disorders or cognitive challenges. These victims do not have to speak for themselves -- the DNA match does it for them. It's a gift and a blessing.  

What are some of the gaps in the public’s (and victims’) understanding of the use of forensic DNA in criminal cases?

Gahn: Victims and the public tend to have the same gaps in understanding DNA and all the forensic sciences. Often, so do police officers and prosecutors, and that is all driven by the "CSI effect." Because of CSI programming, people have unrealistic expectations. They may think there's a lightning-fast turnaround at the crime lab and a tidy, linear progression from the crime scene to the courtroom. But it doesn't work that way, and as part of our trial preparation, we have to explain to victims what to expect. That is why I always stress that training in DNA technology is not only for police and prosecutors, but also for victim advocates. Victims ask these questions, and the advocates need to be aware of the capabilities and limitations of the science so they can answer victims' questions. Once the prosecution mechanism of the case begins, the victim advocate is the person who will generally explain how the science works and how it is used in court. 

From your perspective, what do victims need to understand about DNA testing?

Gahn: DNA testing is a complex, time-consuming process that victims may find very frustrating. We deal with biological substances that are subject to contamination, degradation, and being mixed with other substances. We may have interpretation problems because often there is not a clear, intelligible match. We have backlogs in our crime labs, and there are delays even though we try to push these cases through. We have a time-consuming, precise, peer-reviewed process that involves evidence collection, preservation, and transportation. It has to go through crime lab intake and preliminary tests and on to the DNA testing that is subject to careful quality control. Victims need to be aware of the time and effort that goes into crime scene analysis and evidence testing. It is not as simple as portrayed on CSI- type programming.

What have you learned from working on these cases?

Gahn: What I've learned is -- don't get overconfident. DNA makes associations, but it does not necessarily tell us how, when, or why a crime was committed. As law enforcement officers and prosecutors, we need to step back and keep it in perspective. If we lose sight of basic investigative techniques and other avenues, we will lose cases by relying too much on the DNA tests alone. There is nothing more compelling at trial than a well-prepared, confident, compelling eye-witness who can explain to the jury what happened. Jurors want to see that, and it takes preparation. Police officers still have to collect evidence, find witnesses, investigate fully, and put the case together. We don't want DNA to cause shortcuts for prosecutors and investigators, and we can't rest the whole trial on it.  

What tips do you have for other prosecutors about the use of forensic DNA in their cases? 

Gahn: Get your DNA results, and step back and look at the whole case. Early on, decide what you will use the DNA for. What is the association you want to show to the jury, and how will you explain it? Is it for corroboration, impeachment, identification? Will you use it to show what happened at a crime scene? How will you present it and convey to the jury the meaning of the DNA profile? Think it through in advance, and have an orderly, polished presentation with your goal in mind about the meaning of the profile and how to make it understandable to the jury. 

What advice do you have for victims whose cases involve forensic DNA evidence?

Gahn: As we prepare for trial, victims need to understand that there will be defense attorneys at the trial, and they will try to put some sort of twist on the evidence that we have. They will argue, for example, that the evidence is there because of a consensual act or that it was accidentally there the day before -- some innocent reason. Or they might claim the evidence is there because of some secondary transfer. Whatever the argument, victims have to understand that the defense will have a competing theory about the evidence. The prosecution will then come back and make the proper association for the jury and explain the proper way to interpret the DNA profile and the reasons for the presence of the evidence. Victims need to be ready for that challenge, and victim advocates -- as well as prosecutors -- can help them approach the trial with confidence. And so much of that confidence rests on the gift of forensic DNA.  


Norman Gahn, Assistant District Attorney for Milwaukee County, Wisconsin, is a pioneer in the use of DNA technology in the courtroom. He lectures widely in Wisconsin and throughout the nation on forensic DNA and has been honored for his work by the National Institute of Justice, the Federal Bureau of Investigation, and a number of other organizations.



This project was developed with funding under cooperative agreement 2009-SZ-B9-K010 awarded by the Office for Victims of Crime, Office of Justice Programs, U.S. Department of Justice. The opinions, findings, and conclusions or recommendations expressed herein are those of the contributors and do not necessarily represent the official position or policies of the U.S. Department of Justice.