On a Tuesday morning in Boise, biologist Greg Hampikian is on speakerphone with Christopher Tapp, an inmate at the Idaho State Correctional Institution. Tapp is in prison for a murder he swears he did not commit. Many others think he’s innocent as well—lawyers, journalists, an organization of former judges; even the victim’s mother. No one has fought for him longer than Hampikian, a researcher at Boise State University (BSU). “How’re you doing, Chris?” Hampikian begins. “I’m having a good time at summer camp,” Tapp says gamely. “Although you know me … I’m in the hole again”—solitary. “There was a mutual misunderstanding between me and someone else.” He mentions that he and his wife are getting divorced.

“I’m really sorry Chris, that’s a lot to deal with,” Hampikian says. “You know my offer still stands. I’ll pay tuition for any courses you want to take.” A pause. “You know we got a decision from the court to move forward with a new DNA test. But the court is only allowing us a limited amount and we have to decide which test is the right one.”

I don't think people are evil, but once they are convinced of a story, they protect it. Greg Hampikian

Tapp has been in jail since 1998, serving a 25-years-to-life sentence for the murder of a 19-year-old woman named Angie Dodge; he confessed after a series of lengthy interrogations that several experts have described as coercive. Police found plenty of male DNA at the scene, and it did not match Tapp's. But the prosecutor and jury believed his confession.

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Hampikian, who holds joint appointments in biology and criminal justice at BSU and heads the Idaho Innocence Project, has been helping people like Tapp for more than 20 years. He works with defense attorneys and police around the world, trying to free innocent people by exploiting the power of DNA forensics—or by exposing its pitfalls. As the nation’s only Innocence Project director who’s also a scientist, “he’s absolutely essential to what we do,” said Aimee Maxwell, executive director of the Georgia Innocence Project in Decatur, which Hampikian co-founded.

In Tapp’s case, Hampikian has pushed for the use of a new, controversial DNA technique that he believes may yet identify the real killer and exonerate Tapp. In other instances, such as the notorious Amanda Knox conviction in Italy, he has shown how DNA evidence was false or misconstrued. “Just because it’s DNA doesn’t mean it’s good science,” he says.

Hampikian, 54, who looks a bit like the comedian Bill Maher (but with a beard), is affable, funny, and likes aphorisms. On science and religion: “Theologians are willing to die for their beliefs but scientists are willing to let their beliefs die.”

His involvement with forensic science was a multiyear courtship. He had done research on the Y chromosome in Australia and was teaching at Clayton State University in Morrow, Georgia, in 1993 when an associate of famed criminologist Henry Lee asked him whether science could determine the sex of a person based on traces of saliva left at a crime scene. (It could.) Later, he was so captivated by the story of Calvin Johnson, who was exonerated by DNA in 1999 after spending 16 years in prison for rape, that he helped Johnson write an autobiography, Exit to Freedom. “The idea that you could free someone with a little bit of this snotlike stuff was a notion that seemed thrilling to me,” he says.

Hampikian became a founding member of the board of the Georgia Innocence Project, launched in 2002. In 2004, he moved to BSU, where, in addition to teaching and doing research in genetics, he helped found the Idaho Innocence Project. Along the way, he used DNA to help exonerate at least a dozen wrongfully convicted people in the United States, Taiwan, and Italy. He’s had losses as well. Four of his clients were eventually executed.

DNA evidence is so powerful because it has firm roots in science and is backed by statistics. Analysts focus on 13 or more places in the genome, called loci, where humans are extraordinarily diverse. Each locus contains a “short tandem repeat,” a bit of DNA that is repeated multiple times. The exact number of repeats at each locus varies from person to person and can range anywhere between the low single digits to the mid-50s. Because we get one copy of each chromosome from our mother and one from our father, there are two numbers for each locus, which appear as peaks on an electropherogram, a chart produced by a genetic analyzer.

The chance that two people have the same pairs at all 13 loci is astronomically low. It’s a bit like pulling the handles of two slot machines with 13 cylinders each—all containing dozens of symbols—and hoping they match up right down the line. To reduce the risk of false matches even further, the Federal Bureau of Investigation (FBI) will soon release new guidelines calling for 20 or more locations.

Its accuracy has made DNA evidence virtually unassailable. A landmark report published by the National Research Council in 2009 dismissed most forensics as unproven folk-wisdom but singled out DNA as the one forensic science worthy of the name. Yet in recent years Hampikian and other geneticists have begun to question the technology. Thanks to a series of advances—including the polymerase chain reaction, which can multiply tiny amounts of DNA—it’s now possible to detect DNA at levels hundreds or even thousands of times lower than when DNA fingerprinting was developed in the 1980s. Investigators can even collect “touch DNA” from fingerprints on, say, a glass or a doorknob. A mere 25 or 30 cells will sometimes suffice.

This heightened sensitivity can easily create false positives. Analysts are picking up DNA transferred from one person to another by way of an object that both of them have touched, or from one piece of evidence to another by crime scene investigators, lab techs—or when two items jostled against each other in an evidence bag.

That was the case with Amanda Knox, who was accused of stabbing her U.K. housemate Meredith Kercher to death during junior year in Perugia, Italy. Authorities had accused a local young man named Rudy Guede with sexually assaulting and killing Kercher. The evidence against him was overwhelming—palm prints, fingerprints, and his DNA on the victim and throughout her room—and he was eventually found guilty. But Italian prosecutors also charged Knox and her boyfriend Raffaele Sollecito with murder. Traces of Sollecito’s DNA had been found on the clasp of a bra belonging to Kercher, suggesting that he had taken part in the sexual assault, while a knife in Sollecito’s kitchen drawer showed Knox’s DNA on the handle and Kercher’s DNA on the blade.

Hampikian reviewed the lab’s procedures and data for the defense team. He noted that the bra clasp hadn’t been collected until 46 days after the murder, and not until several crime scene investigators had picked it up, passed it around and then put it back down on the floor to photograph its position—all of which could have caused Sollecito's DNA to end up on the clasp. And although plenty of Knox’s DNA was on the knife handle (she had used it in cooking), the amount of DNA from Kercher on the blade was vanishingly small—less than half the amount the FBI considered valid for testing.

Hampikian's critique was signed by nine other prominent geneticists and made public. Meanwhile, he had his students mimic part of the investigation in Italy. They collected five soda cans from the office of BSU's dean of arts and sciences after lunch and put them in individual evidence bags. Then, without changing gloves, they put five newly bought knives into separate evidence bags. Like the Italians, Hampikian’s group looked for DNA at levels below the FBI-recommended minimum. They found DNA from a member of the dean’s staff on one of the knife blades. Yet that person had not touched or even been in the same room with the knives.

Nevertheless, the Italian court found Knox and Sollecito guilty. They spent 4 years in prison before being freed by an appeals court, only to later be found guilty again. Last spring, after Italian DNA experts reviewed the case, an Italian high court pronounced both innocent.

DNA analysis can become even trickier when a mix of DNA from various potential suspects is found in a single crime scene sample. With a simple sample, analysts look at two sets of peaks at a given locus: one for the victim and one for the perpetrator. With mixtures, they’re looking at bunches of peaks, with no indication of which pairs go together, or which source they came from—aside from those of the known victim. At that point the analysis becomes highly subjective.

Studies have confirmed this. In 2013, geneticist Michael Coble of the National Institute of Standards and Technology in Gaithersburg, Maryland, set up a hypothetical scenario in which a mix of DNA from several people had been found on a ski mask left at a crime scene after a series of robberies. Coble asked 108 labs across the country to determine whether a separate DNA sample, which he posited had come from a suspect in the robberies, was also part of the mix. Seventy-three of the labs got it wrong, saying the suspect's DNA was part of the mix when, in fact, it was not. “It’s the Wild West out there,” Coble says. “Too much is left to the analysts’ discretion.”

Hampikian conducted a similar study using DNA evidence from a real crime: the case of Kerry Robinson, a Georgia man serving 20 years for taking part in a gang rape. The victim had identified a man named Tyrone White as one of her attackers. Indeed, White's DNA matched 11 of the 13 alleles found in a DNA mixture at the crime scene that did not belong to the victim. White confessed; as part of a plea bargain he implicated Robinson, against whom he had a grudge. Robinson had two alleles in common with DNA found on the victim's body, both “at the borderline of detection,” says Hampikian, who testified that Robinson was “absolutely excluded.”

For the study, Hampikian and U.K. psychologist Itiel Dror asked 17 analysts at a reputable U.S. lab to interpret the electropherograms from the case, without specifying their origin. Twelve of the analysts excluded Robinson from the mixture, four said they could draw no conclusion, and only one said Robinson’s DNA was in the mix. Hampikian's lab also tested the DNA of four employees from a local TV news station; all four had at least two alleles in common with the crime scene, like Robinson. One of them, a 26-year-old white female, had three.

“Greg was fantastic,” says Robinson’s attorney, Rodney Zell of Zell & Zell law firm in Atlanta. “He knows his stuff and is a great witness. But it’s hard for a judge to overturn a 15-year-old conviction.” Last summer, an appeals court judge ruled against Robinson’s appeal. Zell appealed that decision to the Supreme Court of Georgia, where it’s now pending. Tyrone White has served his reduced sentence and walked free.

“I don’t think people are evil,” Hampikian says, “but once they’re convinced of a story they protect it.” On the phone with Chris Tapp, he's explaining why it’s been so hard to overturn his conviction. Yet Hampikian thinks recent scientific advances in DNA forensics could still free him.

Idaho Falls police detained Tapp because he was a friend of their prime suspect, who was eventually released. Working on the theory that several people must have attacked Angie Dodge, police offered Tapp immunity if he would reveal the name of the other assailant. Over the weeks, Tapp gave them several dozen names, but DNA ruled out each new suspect. Eventually police withdrew Tapp’s immunity, leaned on him until he confessed, and charged him. They said Tapp held Dodge's arms down while one or two other men assaulted and killed her.

In order to mount an appeal, Tapp’s lawyers must produce evidence that was not available at the time of the trial. One possible approach is a search process called familial DNA. Even if police can't match DNA from a crime scene to FBI's national DNA database, loosening their search criteria may lead to a so-called partial match—possibly a relative of the perpetrator, which could provide an investigative lead. Civil rights advocates object to this procedure because it means that simply being related to an offender can make you a person of interest; Maryland and the District of Columbia have banned the practice. But it has solved some important cases, including that of a Californian serial killer nicknamed the Grim Sleeper, who’s now on trial.

In the case of Angie Dodge, Idaho Falls police checked the crime scene DNA against FBI’s database and did not find a match. (It’s not entirely clear whether they searched for a partial match as well.) At Hampikian’s suggestion, they also conducted what he calls a "genealogical search." Police had a forensics lab produce a profile of 35 loci on the Y chromosome from semen found at the crime scene; then they obtained a subpoena to have Ancestry.com, the largest genealogy company in the world, compare the profile with a Y chromosome database that the company had bought in 2012 from a Mormon genealogy foundation.

One man, a Mississippi resident with no criminal record, matched at 34 of the 35 locations. The man, who had submitted a cheek swab at the urging of his Mormon pastor, was too old to fit the killer’s profile. But police took an interest in one of his sons, Michael Usry, Jr., a 36-year-old filmmaker in New Orleans, Louisiana, who produced gory murder films and who had passed through Idaho near the time of the murder in 1996. They obtained a court order to sample Usry's DNA in 2014—but the test results excluded him. Tapp’s attorney, John Thomas, says he is looking into legal approaches to expand the familial searching. (Usry, meanwhile, has become so interested in Tapp’s case that he announced plans to make a film about it.)

Last spring, Thomas got court permission to try another avenue: have a laboratory do “touch DNA” testing of swabs that had been taken from Dodge's hands. If Tapp's DNA isn't found in them, that would disprove the prosecutors’ claim that he held her down while she was killed. “We’re going to keep attacking the state’s theory piece by piece,” Hampikian says. He worries that the court might reject the new evidence, as they have in previous appeals. He worries that Tapp won’t go free until his parole in 2026. But he puts on a brave phone voice for him as he explains the options.

Tapp says: “I appreciate everything you guys are doing, and thank …” but the line cuts off before he can finish his sentence.