Under the Microscope – Lisa Mertz

Within the past 15-20 years, successful technological advances in forensic science have solved many cold cases. The increase in the number of possible items that might presently be tested as evidence in criminal matters, in general, has led to the need for collaborative efforts among the New York City DNA Laboratory, the New York City Police Department, and the New York City District Attorney’s Offices.


In her presentation at ISHI, Lisa Mertz (Criminalist Level IV, Senior Supervisor, and Cold Case Liaison at the OCME) will present examples of successful cold cases tested in New York City, despite the existing constraints, and will highlight how advances in technology and a multiagency team approach have resulted in their resolution. We sat down with Lisa to discuss how mindsets need to change when working with cold cases, working with difficult samples, and her advice for other agencies.



Lisa, thanks for talking with us today! How did you get involved in working with cold cases? Does your mindset change when working with older cases versus active cases?

I started working in cold cases when I became a part of the high sensitivity team at the OCME over 6 years ago.  This was prior to the validation of the more sensitive DNA amplification kits that we have today.  At that time, there was a need for a group of specialized individuals with casework experience to be involved in lower level DNA samples.  As part of this team, we often went back to older cases to see what was possible in terms of DNA testing and lower level DNA samples.

When working on cold cases, one’s mindset needs to change since you often need to look outside the box.  At the same time however, you need to be realistic concerning what is possible.


Can you explain the process of re-examining a cold case? How are cases chosen? How do you determine which evidence to test?

Re-examining a cold case involves a teamwork approach.   Prior to DNA testing, discussions are had with the District Attorney’s office and the cold case Detectives concerning possible evidence to test.  Everyone’s input is needed in order to obtain information as to what items are the most probative, as well as those items that are most likely to give you a DNA profile.  Often this discussion is accompanied by crime scene photos and old serology laboratory reports.

Cases are chosen based upon several factors, including renewed interest in the case, through a family member or a cold case detective, new investigative lead, or present-day case that has a DNA profile which matches and hits to an older cold case.


What’s the most impossible sample you ever had to work with?

Many cold case samples can be difficult to work with since they are often degraded or compromised due to age of the samples or the circumstances in which they were collected.  Most cold case samples were processed at a time when developing a DNA profile was not even thought to be possible.  The most frustrating samples as a DNA analyst, however, are those samples that you feel should give you a DNA profile but are unsuccessful in doing so.  These are samples that traditionally are thought to have a lot of DNA for processing and would typically give you a DNA profile.   One example is a blood sample left behind at a crime scene.


Have you picked up any tips and tricks along the way that other labs could use? (For example, when working with older, degraded samples, how does your process differ compared to fresher samples from more recent cases?) 

A tip or trick that I believe is helpful to laboratories that are processing cold cases is the thought that you can never obtain too much information about the case you are working on.  The information you gather may seem frivolous at first but can make or break the case.  For example, just knowing where to sample an item of clothing based upon how it was worn by the victim can be the difference between solving the case and the case remaining cold.  Having this viewpoint has led to a great deal of success when processing items such as ligatures that were used to bind a victim prior to or after his or her death.  By seeing the crime scene photos and gathering as much information as possible, we can get a better idea as to where on the ligature one should sample for DNA from a foreign source.  Without this information, however, only the DNA profile from the victim might be obtained.


How have technological advances over the years assisted in closing older cases?

DNA testing has greatly evolved over the years since it was first discovered in the 1950s.  The sensitivity of DNA testing has led to greater success with smaller quantities of DNA, thereby increasing the ability to get profiles from only a few cells.  This has led to an increase in successful profiles obtained from cold cases.  As PCR kits have increased the number of DNA locations that can now be tested, the odds of obtaining a profile with enough data suitable for comparison have also increased.  Without DNA testing we would still be in the dark ages of serology testing (such as ABO blood type testing) and would be unable to individualize the information obtained or statistically associate an individual to crime scene evidence to aid in convictions in the courtroom.


Can you describe the multiagency approach that New York City has taken when addressing cold cases? Do you have any suggestions for other agencies looking to tackle their cold cases?

The multiagency approach that New York City uses to process cold cases involves members of the Forensic Biology team, who are experienced in casework, working together with specialized cold case Detectives and the District Attorney’s office to come together as a collaborative team to solve a case that has gone cold.  The Detectives bring to the team their knowledge of the case and the crime scene, as well as vital crime scene photos, if they exist.  The District Attorney’s Office brings to the team its knowledge of the law and what is possible in terms of prosecuting a suspect should he/she be discovered on an item of evidence.  The DNA laboratory then uses all this information to then discuss what is possible in terms of DNA testing.  This collaborative effort is seen as each member being a piece in a complicated puzzle.  If any one piece is missing, it is difficult to see the full picture.


Are there any cases that have really stuck with you? If so, would you be able to describe one?

The cold cases that I have been involved with through the years are in their own way important and have stayed with me.  I think the most difficult cases are those where a DNA profile is developed and uploaded into the CODIS system, but the case remains unsolved.  After all of the work you put into obtaining a DNA profile, you are left wondering whether the suspect is dead?  Has the suspect moved across international borders?  Or is the suspect still out there and has yet to be discovered?  It is reassuring to think, however, that his or her profile is stored in the CODIS database and will hopefully one day solve the case.


What tips would you give to someone who is just starting out in the field of forensics, or what is the best advice that you’ve received?

I think the best advice I can give to someone who is just starting out in forensics is the knowledge that forensics is not easy. It is not as portrayed on TV.  DNA is not always the answer and sometimes a case will remain unsolved.  I think one should trust his or her instincts concerning what is possible in terms of DNA testing.  He or she will also hopefully not forget, when processing hundreds of samples in a career, that there is a victim behind the samples and often a family who is desperate for answers.  This enables the work to be about science serving justice, our mission at the OCME.


As we celebrate the 30th anniversary of ISHI, do you have any predictions for what the future holds, or do you have any fond memories of using older technologies/techniques?

I think the future will be very exciting for forensics.  Advancements, such as whole genome testing with next generation sequencing and genealogy will only propel forensics into a world of less doubt.  I do, however, miss the art form that was pouring a perfect 377 sequencing gel.