DNA has become an increasingly important tool for law enforcement. Police can collect DNA from crime scenes and test it against samples from suspects or DNA databases to try to identify perpetrators. But how does the process work? Here’s an overview of what police do with DNA evidence.
DNA Collection at Crime Scenes
Police may collect DNA evidence from a wide variety of sources at crime scenes including:
- Blood, semen, or saliva stains
- Hair follicles or skin cells
- Cigarette butts, chewing gum, or drinking bottles that may contain the perpetrator’s saliva
- Items the perpetrator touched that may contain skin cell DNA such as doorknobs or tools
Crime scene investigators are trained in collecting DNA evidence and use protective equipment like gloves and face masks to avoid contaminating samples with their own DNA. The DNA evidence is carefully packaged to preserve the integrity of the sample.
DNA Samples from Suspects
Police can obtain DNA samples from suspects in a criminal investigation through:
- Court-ordered DNA testing – Police can get a search warrant or court order to compel a suspect to provide a DNA sample through blood testing, cheek swabbing, etc.
- Voluntary submission – A suspect may voluntarily submit their DNA to try to clear their name.
- Surreptitious DNA samples – Police may collect DNA from items a suspect touched without their knowledge such as cigarette butts or coffee cups.
- Arrestee DNA laws – Some states allow DNA to be collected from all felony arrestees.
Police typically obtain a DNA sample using a buccal swab, rubbing a cotton swab on the inside of the cheek to collect skin cells. The DNA is extracted from the cells in a lab.
DNA Analysis
Forensic scientists analyze DNA evidence from crime scenes and suspects and make comparisons. Traditional DNA profiling looks at 20 specific locations (loci) in DNA that are unique to each person (except identical twins). By analyzing the lengths of DNA segments at each locus, a DNA profile can be generated.
If DNA from a crime scene matches the profile from a suspect’s DNA sample, this suggests the suspect’s DNA was present at the crime scene. Match statistics can demonstrate how significant the match is.
More advanced DNA analysis methods like mitochondrial DNA testing or Y-STR profiling may also be used in some cases.
DNA Databases
DNA profiles from crime scenes, convicted offenders, and arrestees are often uploaded to and compared against DNA databases including:
- National DNA Index System (NDIS) – The U.S. national DNA database with over 12 million profiles.
- State databases like California’s CAL-DNA database.
- Local city and county databases.
Matches made between a crime scene DNA sample and a database profile can provide investigators with leads on potential suspects.
CODIS
The FBI’s Combined DNA Index System (CODIS) integrates DNA databases at the local, state, and national levels and allows for comparisons across jurisdictions. CODIS has the following different indexes:
- Convicted Offender Index – DNA profiles of people convicted of crimes.
- Arrestee Index – DNA profiles of people arrested for crimes.
- Forensic Index – DNA profiles from crime scene evidence.
- Missing Persons Index – DNA of missing persons and unidentified remains.
- Relatives of Missing Persons Index – DNA profiles of relatives to identify missing people.
CODIS uses computer algorithms to search across indexes for matches between casework and offender profiles or across casework profiles that may be linked.
DNA Evidence in Court
Prosecutors can present DNA evidence in court to demonstrate that:
- The defendant’s DNA was present at the crime scene.
- The defendant’s DNA profile matches DNA from the victim, crime scene, or other relevant sources.
- Match statistics show the significance of the DNA match.
- A DNA expert testifies to the reliability of DNA testing methods.
Defense lawyers may challenge DNA evidence on grounds of contamination, improper laboratory procedures, or inaccurate statistics claims. But DNA evidence can be very persuasive to juries when properly collected and analyzed.
Expanding Use of DNA in Policing
Law enforcement is finding new applications for DNA analysis such as:
- Familial DNA searching – Looking for partial matches in offender databases that may indicate a relative of the perpetrator.
- Genealogy databases – Uploading crime scene DNA to public genealogy websites to identify possible suspects through relatives.
- DNA phenotyping – Using DNA to predict physical characteristics like ancestry, eye color, and face shape to generate leads.
- Microbiome analysis – Testing bacteria in body areas like the gut, mouth, and skin to link individuals to locations.
But these emerging uses raise privacy, consent, and ethical issues that are still being debated. Regulations vary across states.
DNA Sample Retention
Laws on how long police can retain DNA samples and records from those not convicted of any crime also differ between states:
- Some states like California require all DNA samples and records be destroyed if a conviction is overturned or charges dropped.
- Other states like Louisiana place no limits on police retention of DNA samples from arrestees.
- Maryland retains DNA for only 3 years if no conviction.
Privacy advocates argue for limiting retention times citing privacy risks with indefinitely storing DNA profiles from those never charged or acquitted of crimes. But law enforcement wishes to expand DNA databases to solve more cases.
Conclusion
Police use DNA as a powerful investigative tool, collecting it from crime scenes, suspects, and databases to identify perpetrators through genetic matching. But appropriate regulations are still needed around emerging forensic DNA analysis methods and sample retention rules to protect privacy rights. When DNA evidence is properly handled, it can provide compelling evidence of guilt or innocence in court. Expanded DNA databases help ensure serial offenders are identified, but should balance crime-solving value with civil liberties concerns.