Every cell within every living creature contains DNA material. That material carries instructions that dictate everything from how tall you'll be to what diseases you may develop, and it's unique to you. Forensic scientists can find it in biological material left on a crime scene or body, like hair, saliva or even skin tissue.
Through DNA profiling, also known as DNA fingerprinting, scientists analyze that material and create a chart on which variations show up at different locations. These are visualized as peaks and are translated into numbers that can be matched with the DNA of other suspects or with material from missing people.
Over the years, DNA has become one of forensic science's most powerful tools, helping to identify suspects and victims, convict the guilty and exonerate the innocent. DNA science and technology have grown so advanced that a mere touch can link someone to a crime scene.
"When I told people in 1977 in high school that I wanted to be a forensic scientist, they literally thought I was talking about voodoo and witchcraft," said Jenifer Smith, director of the District of Columbia's Department of Forensic Sciences and a former FBI special agent. "What DNA did in the late '80s and early '90s was sort of bring a more objective science ... cool technology, molecular biology. It gave almost this credence to forensics, because now, it looks more like a science."
Dwight E. Adams was the first FBI official to testify on DNA evidence in the United States and helped oversee the FBI's establishment of DNA profiling rules and guidelines for labs across the country. He called DNA "the single greatest advance in forensic science."
"The technology has improved tremendously since 1988 when it would take us 6 weeks to perform one test," Adams wrote in an email. "Now, laboratories are performing the test in about 24 hours and able to work with samples that we could only dream about in the early days."
Still, forensic science and DNA profiling aren't foolproof.
During his years in the White House, President Obama implemented several initiatives to improve forensic evidence gathering
. In a 2017 Harvard Law Review
article, he said they were sparked by lingering concerns from a 2009 National Academy of Sciences
report, along with a rash of "high-profile exonerations of wrongfully convicted individuals that indicated that testimony exceeded the scientific capabilities of the technique."
"Contrary to the perception of TV dramas, forensic science disciplines are subject to varying degrees of uncertainty and misinterpretation," Obama wrote.
Forensic evidence pinning a suspect to the scene of a crime can be powerful in the courtroom. But scientists agree that when investigators testify about that evidence, they haven't always emphasized to the jury that science can make mistakes, such as DNA contamination in labs or DNA transferred from one crime scene to another.
One of Obama's initiatives launched a review of FBI testimony in cases. Another brought together scientists, law enforcement officials, judges and lawyers to create the National Commission on Forensic Science. Both of these initiatives were ended in April
by Attorney General Jeff Sessions, who said the Trump administration would seek its own path toward improving criminal investigations under a new task force
Some investigators said that over the years, funding has not kept up with the demand for evidence analysis, and labs are overwhelmed.
"Forensic science has been dealing with a resource problem," said former investigator John M. Collins Jr., whose Forensic Foundations Group works to educate lab technicians.
Indeed, crime labs around the country now process over 3 million requests per year, one-quarter of which is DNA profiling, according to the Bureau of Justice Statistics
Here are a few notable cases in which DNA evidence made a mark.
30 years since an early test
In 1986, authorities in Leicester, England, were investigating the rapes and murders of two young women. A suspect confessed to the crime involving one woman but not the other. Convinced the two crimes were linked, investigators sought the help of Dr. Alec Jeffreys, a geneticist who developed techniques to visualize bands of DNA in his lab.
With Jeffreys' help, authorities analyzed the DNA of hundreds of men living near the crime but found no match. But the analysis also cleared the man who had confessed. In 1987, authorities found that local baker Colin Pitchfork had avoided taking the test. His sample was a match for both killings, and under pressure from DNA evidence, he confessed to the crimes.
Exonerated by DNA
In 1989, Gary Dotson
became the first person exonerated because of DNA testing. He'd been behind bars for over a decade after a woman accused him of rape in 1977.
Investigators used blood-type and hair analysis to convict him, but he appealed for years, until DNA testing could be applied to material still held from the case. DNA cleared him, and he won his release. Testing linked the evidence to the accuser's then-boyfriend, and the woman admitted she'd made up the rape.
DNA science was slowly becoming more precise. And few years after Dotson's release, in 1994, the FBI expanded its Combined DNA Indexing System, known as CODIS
, which allows law enforcement officials and crime labs to share and search through thousands of DNA profiles. It also sets guidelines for collection and analysis of DNA. It's helped in more than 350,000 investigations.
A trial on television
In the 1995 trial of star athlete O.J. Simpson, a huge television audience followed along as the defense picked apart forensic evidence gathered by the state, particularly a bloody sock, knife and glove. The defense team raised questions about whether the DNA could have been contaminated.
Ultimately, those questions made a difference: Simpson was acquitted in the June 1994 killings of his ex-wife, Nicole Brown Simpson, and her friend Ronald Goldman.
The case helped civilians understand that DNA and forensic science could be flawed. Marcia Clark, the prosecutor in the case, has said police mishandling of the evidence and shoddy forensic collections created a distrust of the Los Angeles Police Department.
Simpson is imprisoned in Nevada in a separate case and is up for parole this year.
A 20-year case
Early on, scientists needed significant amounts of DNA in order to analyze it, which prevented its use in many cases. But that changed over time.
Starting in 1982, authorities in Seattle searched fruitlessly as a serial rapist and murderer killed dozens of women and buried their bodies along the Green River in Washington state. Many were prostitutes 16 to 36 years old.
The case went cold, but in 2001, authorities were able to review old evidence using a technology called PCR, or polymerase chain reaction. PCR takes tiny amounts of DNA, previously nearly impossible to analyze, and copies it over and over. Authorities matched DNA from the victims' bodies to one of their prime suspects, Gary Ridgway.
Under pressure from DNA and other forensic evidence, Ridgway confessed to 48 counts of murder. (The story is being retold by HLN's "Beyond Reasonable Doubt.
Questioning how far DNA can go
After the 2007 killing of 21-year-old British student Meredith Kercher in Italy, American Amanda Knox and her boyfriend, Raffaele Sollecito, were convicted of murder in 2009. But there was an outcry from scientists and investigators in the United States. They were suspicious of DNA collection throughout the crime scene and questioned, among other things, the finding of Sollecito's DNA on a small part of Kercher's bra.
After years of legal back and forth, Knox's and Sollecito's murder convictions were overturned in 2015. Another man, Rudy Guede, was convicted in Kercher's death and remains in prison.
As DNA technology became more sensitive, its uses expanded and demand grew -- but the tests can't always keep up.
"What happened in the Amanda Knox trial, in that investigation, is symptomatic of another issue, and that is that both the public and prosecutors have been pressuring ... and I suppose defense attorneys, the whole system ... is pressuring labs into pushing the envelope of what these tests can do," said Dan E. Krane, a biology professor at Wright State University who's reviewed cases for defense teams for decades, including the Knox case. "The crux there, the central issue, is ambiguity."
Forensic analysts give a statistical analysis of whether DNA can pinpoint the suspect in the case, but Krane and many others argue that analysts could go further to explain the possibility of error to the jury. DNA's presence on a scene, Krane said, does not indicate when or how it got there.
There's work to do on educating jurors and the public about DNA's limitations, but, Krane said, it remains "the gold standard of forensic science. It doesn't mean that there isn't room to improve that gold standard, but all the rest of forensic science, and I mean everything -- fingerprint, hair and fiber, handwriting, blood spatter, gunshot residue, you name it -- everything else needs to aspire to have that same sort of scientific rigor that is now in play for DNA profiling."