Post-conviction DNA testing is the forensic analysis of biological evidence after someone has already been found guilty and sentenced for a crime. Its primary purpose is to determine whether the convicted person is actually innocent, typically in violent felony cases like sexual assault and homicide where physical evidence was collected but never tested, or where older technology produced less reliable results than what’s available today. In 2024 alone, 147 people were exonerated in the United States after losing an average of 13.5 years to wrongful imprisonment.
Why Old Cases Get Retested
Many convictions, especially those from the 1980s and 1990s, were secured before modern DNA technology existed. Evidence like blood, saliva, hair, or sexual assault kits may have been collected at the time but either never analyzed or tested with methods far less precise than what laboratories use now. In other cases, a conviction rested heavily on eyewitness testimony or confessions that later came into question, and physical evidence that could confirm or contradict the verdict sat untouched in storage.
Today’s standard method uses short tandem repeats (STRs), sections of DNA where a short sequence repeats a variable number of times. Because the number of repeats differs from person to person, STR analysis can distinguish between individuals with extremely high accuracy. Newer techniques go even further. In cases involving mixed biological samples, such as sexual assault evidence containing DNA from both a victim and an attacker, specialized markers can isolate the male contributor’s profile even when it’s present in tiny quantities. These advances mean that evidence once considered too degraded or too contaminated to test can now yield usable results.
Who Qualifies for Testing
Not every convicted person can request post-conviction DNA testing. Federal law, established through the Innocence Protection Act of 2004 (Title IV of the Justice for All Act), sets out specific requirements. The applicant must assert under penalty of perjury that they are actually innocent. If the conviction followed a trial, the identity of the perpetrator must have been a contested issue. And the proposed DNA testing must be capable of producing new material evidence that would raise a reasonable probability the applicant did not commit the crime.
Most states have their own statutes with similar frameworks, though the details vary. Some states impose time limits on when a motion can be filed. Others require the applicant to show that the evidence wasn’t previously tested through any fault of their own. The common thread across jurisdictions is that testing must have the potential to change the outcome. A case where DNA evidence wouldn’t meaningfully speak to guilt or innocence, say a financial fraud conviction, won’t qualify.
How the Process Works
The process begins with a motion filed in court requesting access to specific biological evidence. This isn’t a general request. The person filing must identify exactly which evidence they want tested, explain why that evidence matters to the question of guilt or innocence, and provide the last known location of the evidence. The motion needs to demonstrate, requirement by requirement, how the case meets the state or federal testing statute.
If the court grants the motion, the evidence is sent to a forensic laboratory for DNA analysis. This stage can take weeks to months depending on the condition of the evidence and the lab’s backlog. Once results come back, there are three basic outcomes: the DNA matches the convicted person (supporting the original verdict), the DNA excludes the convicted person as the source (supporting innocence), or the results are inconclusive, often because the sample was too degraded.
When results point to innocence, the next step is filing a motion for a new trial based on newly discovered evidence. If the court finds that the DNA results establish, by a preponderance of the evidence, that a new trial would likely result in an acquittal, it must grant that motion.
What the Numbers Show
A study by the Urban Institute examined what happens when post-conviction DNA testing is actually performed. In homicide and sexual assault cases combined, testing eliminated the convicted person as the source of incriminating physical evidence about 5 percent of the time. When sexual assault cases were looked at separately, that figure rose to between 8 and 15 percent. These numbers mean that in the vast majority of cases, DNA testing confirms the original conviction. But the cases where it doesn’t represent real people serving time for crimes they did not commit.
The federal program that funds post-conviction testing, run through the Bureau of Justice Assistance, acknowledges both sides of this equation. Exonerations that correct injustice are significant outcomes. But cases that go through careful review and ultimately confirm guilt also serve the public interest by demonstrating that justice was fairly applied.
The Role of DNA Databases
When testing excludes the convicted person, the resulting DNA profile doesn’t just sit in a file. It gets run through CODIS, the FBI’s Combined DNA Index System, which contains millions of DNA profiles from convicted offenders, arrestees, and unsolved crime scenes. A match in CODIS can identify the actual perpetrator, generating what’s called an “offender hit” that gives investigators a name. Even without a direct match to a known person, the profile might link to DNA found at other crime scenes, connecting cases that were never previously associated.
Once CODIS identifies a potential match, the laboratories involved exchange information to verify it. A confirmed database match can then be used to establish probable cause for obtaining a fresh DNA sample from the identified suspect, building a new case against the person who actually committed the crime.
Evidence Preservation Rules
None of this works if the biological evidence no longer exists. Federal law addresses this directly: biological evidence in a federal criminal case must be preserved for the entire duration of the defendant’s imprisonment. Destroying, altering, or tampering with preserved biological evidence to prevent DNA testing is a felony punishable by up to five years in prison.
There are limits to this protection. The preservation requirement ends when the convicted person is released from prison, even if they remain on supervised release or parole. And the government can destroy evidence if it notifies the defendant after all direct appeals have been exhausted and the defendant fails to file a testing motion within 180 days. One significant gap in the law: if evidence is lost or destroyed in violation of the preservation rules, that failure does not by itself entitle the defendant to any legal relief. Government employees who violate the rules face disciplinary sanctions from their agencies, and potentially criminal charges, but the convicted person whose evidence is gone has no automatic remedy.
State preservation laws vary widely. Some require evidence to be kept for the length of the sentence, others for a fixed number of years, and some have no mandatory preservation requirements at all. For anyone considering a post-conviction DNA petition, the survival of the physical evidence is often the first and most critical question.
What Happens After Exoneration
The collective toll of wrongful convictions is staggering. In 2024, the 147 people exonerated across the country had lost a combined 1,980 years to prison. Exoneration itself is only the beginning of a long process. The formerly convicted person must rebuild a life after years or decades of incarceration, often with limited financial resources. Some states offer compensation for the wrongfully convicted, while others provide little or no support.
For the justice system, each exoneration also means a crime with an unresolved perpetrator, someone who may have continued offending while an innocent person sat in prison. Post-conviction DNA testing serves both purposes simultaneously: freeing the wrongly convicted and, through database matching, identifying people who should have been held accountable from the start.