The Mechanics of Investigating a DNA “Partial Match”

New York City medical examiner’s office (Photo NY Times)

by John Paolucci

DNA as a biometric identifier has come a long way since it first caught the public’s eye in 1995 during the O.J. Simpson trial.  Its applications now encompass low level or “property” crimes.

In the intervening years, Combined DNA Index System (CODIS) databases have grown exponentially, and the technology for collecting and analyzing DNA evidence continues to evolve with no end in sight.  When an investigator receives word that a clean profile from evidence collected at a crime scene has been loaded into the CODIS databases and produced a match to a convicted offender, he receives a copy of a report explaining that this combination of alleles would be expected to be found in approximately “1 in greater than 1 trillion people.”   So if we had 143 planet earths, you might find that same profile repeated in another individual.  

As I’ve explained to over a couple thousand NYPD personnel and many outside agencies whom I’ve trained, a DNA convicted offender match is an investigative lead, and NOT probable cause.

That being said, what do you do with a DNA “Partial Match”?  Let me start with a simple explanation of what a partial match is:

A partial match occurs when an unknown crime scene profile (DNA profile produced from evidence collected at a crime scene) is uploaded into a Combined DNA Index System, and the database recognizes that the alleles, while not a “match” to a convicted offender, exhibit familial characteristics of a convicted offender.  (In New York City, the Office of Chief Medical Examiner (OCME) Department of Forensic Biology (FBio) is where CODIS managers are located, not the NYPD. The OCME, for some years prior to this experiment, had maintained data on these partial matches, though this data was not disseminated to investigators until early 2011, when it was agreed that a partial match had potential to be an investigative lead.)

The identity of the offender in CODIS partially matching the unknown crime scene profile was very sensitive information.  Let’s imagine a one-time, low level offender who paid his debt to society, which included a DNA exemplar contribution to CODIS, started a new life, got a good job, got married, had kids, and is  sitting at the dinner table when the doorbell rings.  A team of detectives, suits, shields displayed, tell him in a not-so-inside-voice, “Someone in your family raped a woman in the 6th precinct, and we want to know who.” This is of course a worst case scenario, and not anything likely to happen, but certain filters had to be created and implemented to guarantee that this would not occur.

OCME already had over 75 partial matches in backlog when NYPD decided to proceed with exploiting these leads.  It was determined that this would be a venture that necessitates the involvement of three agencies, NYPD, OCME and District Attorneys’ offices (DAO).  New forms were needed along with a strict protocol to determine if the offender’s name would benefit the case.  OCME had the offender’s name, NYPD had the open case from which the unknown crime scene profile was collected, and DAO would rule on whether or not it was worth proceeding with the investigation from a prosecutorial viewpoint and whether or not the statute of limitations had expired.  The latter involved a little more than a simple math equation, since some sex offenses had a “John Doe” indictment, which means that the DNA profile produced from the crime scene or rape kit was indicted, stopping the clock on the statute of limitations.

The first step is for the OCME to release an “Inquiry” form with all available case identifiers, laboratory numbers, complaint numbers, storage numbers, incident dates, type of sample that generated the partial match, CODIS database in which the match was identified (Local, State, National) and type of offense being investigated. The name of the offender is not included in this form. The form was simply to provide NYPD investigators with enough information for it to decide whether or not it would be beneficial to proceed with the investigation and request the offender’s name.  Some cases may have been closed to an arrest, or the victim has since recanted the allegation, or perhaps the complainant is now deceased or moved away and is unwilling to proceed with the investigation.

The OCME would forward this form to my unit, where we would verify the accuracy of the data, and locate the necessary data to complete any fields that were blank, such as a complaint number.  This was often difficult, as we immediately discovered, because many of these cases occurred 20 years ago.  Storage numbers for the evidence would be used to locate the “voucher” or “invoice” on which the evidence was documented.  Once, or if the invoice was located, we could identify the complaint number which contains the precinct of occurrence and is essential in locating the actual case file.  The Chief of Detectives’  “Forensic Initiative” was begun in 2007 and, among many other things, provided for more detailed forms and standardized entries on evidence documentation, with my unit performing quality assurance reviews of the submissions.   Documentation standards were less stringent prior to this revolution, and collating all the missing data required a great deal of effort.

Once all available data was retrieved, the DAO would be updated and we would forward a questionnaire to the investigating unit, requesting a glut of information about the case.  We would need to know if there were witness statements, and if so, were witnesses still available and willing to proceed.  Conversely, was there a witness who at the time was reluctant to provide information, but is now willing to cooperate?  Is there other forensic evidence in the case?  Laboratory capabilities, having come a long way since the occurrence of the incident, could now possibly make use of an item of evidence that at the time was of no value, possibly negating the necessity to proceed with the partial match.  As you can imagine, the need for the offender information could easily diminish depending on these variables and in many cases there would be no need to release this information at all.  Of course, we would also need confirmation from the DAO that it would proceed with the prosecution prior to re-interviewing witnesses and complainants and submitting new evidence for laboratory analysis.

When the investigating unit had finished its assessment, the determination as to whether or not it would like the offender information would be returned to the Forensic Investigations Division.  Responses in both the affirmative and the negative were reviewed, which at times resulted in an executive level conferral when the investigating unit’s determination was in question.   Once resolved, and when the decision was made to proceed with the investigation, and that the partial match could provide assistance, it was time to request the offender’s information.

There were some notions that we couldn’t help but ponder in regards to these offenders, such as, “ Is this someone who lives near the location of occurrence, and could this person have been present when the offense occurred and not deposited DNA at the scene?”  This is merely speculation, and by no means to be included as part of the investigation, but some interesting ideas were tossed around while brewing yet another pot of fortified cop coffee.  The partial match program was taken very seriously, and the highest levels of integrity and discretion were utilized.  It is interesting to consider the future of DNA, and the plethora of data that is available and not used in crime fighting.  Could we one day identify that phenomenon every good cop can sense while trying to raise his standard of proof to “probable cause,” known as the “Perp Gene”?


About the Author:  John Paolucci is a retired detective sergeant from NYPD who worked his last eight years in the Forensic Investigations Division, four of them as a Crime Scene Unit supervisor.  He was the first to command the OCME Liaison Unit, and developed a strong alliance between the OCME and NYPD.  He also worked as a narcotics undercover and patrol officer in the housing projects of the South Bronx. He is currently the president of Forensics 4 Real Inc., training students and law enforcement in forensic evidence and crime scene investigations.  He also performs consultations and on air commentary for movie and television writers, directors and developers of ‘real crime’ shows as well as dramas, is an expert witness and performs investigations such as traveling to South America to exhume a body for DNA collection.