DNA's dirty little secret: a forensic tool renowned for exonerating the innocent may actually be putting them in prison.

• Author: Bobelian, Michael

[ILLUSTRATION OMITTED]

Three days before Christmas 1972, a twenty-two-year-old nurse named Diana Sylvester wrapped up her night shift at the University of San Francisco Medical Center and made her way to her apartment, halfway between the hospital and Golden Gate Park. She arrived around 8:00 a.m. and set her newspaper and purse on the kitchen table. A few minutes later, Sylvester's landlord, Helen Nigidoff, heard loud thuds and screams emanating from Sylvester's unit upstairs. With her apron still on, Nigidoff rang the doorbell before opening a door leading up to Sylvester's apartment, where she came face-to-face with a stranger. "Go away," he growled angrily. "We're making love." As Nigidoff raced downstairs to call the police, the man ran out of the building holding a denim jacket over his face.

When the officers arrived a half hour later, they found a gruesome scene. Sylvester lay motionless next to the Christmas tree on her living-room floor, her mouth unnaturally agape, blood oozing from her chest like molten lava. An autopsy revealed that Sylvester's attacker had forced her to perform oral sex and then strangled her, before plunging a knife into her chest two times. One stab pierced her heart. The other tore through her left lung, drowning her in her own blood.

Police immediately scoured Sylvester's apartment and questioned the landlady, who offered a description of the assailant: white, medium height, and heavy-set, with curly brown hair and a beard. But neither these details nor the bits and pieces of evidence they collected in the months-long investigation that followed were enough to pinpoint the culprit. The few leads investigators turned up fizzled, and the case went cold.

Then in early 2003 the San Francisco Police Department, which had received a grant to use DNA technology to crack unsolved crimes, dug Sylvester's case file out of storage and discovered a slide with sperm that had been swabbed from Sylvester's mouth after her death. The sample was badly deteriorated and contained less than half the DNA markers that are normally used to link a suspect to a crime. But investigators ran the profile through California's DNA database and turned up a match: an ailing seventy-year-old man named John Puckett, who had a history of sexual violence. There was no other physical evidence linking him to the crime. But Puckett was arrested, tried, and eventually convicted based mostly on the DNA match, which was portrayed as proof positive of his guilt--the jury was told that the chance that a random person's DNA would match that found at the crime scene was one in 1.1 million.

If Puckett's were an ordinary criminal case, this figure might have been accurate. Indeed, when police use fresh DNA material to link a crime directly to a suspect identified through eyewitness accounts or other evidence, the chances of accidentally hitting on an innocent person are extraordinarily slim. But when suspects are found by combing through large databases, the odds are exponentially higher. In Puckett's case the actual chance of a false match is a staggering one in three, according to the formula endorsed by the FBI's DNA advisory board and the National Research Council, a body created by Congress to advise the government and the public on scientific issues. But the jury that decided Puckett's fate never heard that figure. In fact, his lawyers were explicitly barred from bringing it up.

Over the past quarter century, DNA evidence has transformed criminal justice, freeing hundreds of innocent people and helping unravel countless crimes that might otherwise have gone unsolved. It has also captivated the public imagination: the plots of popular TV crime shows often hinge on the power of DNA to crack impossible cases, which has helped to give this forensic tool an air of infallibility--a phenomenon known in criminal justice circles as "the CSI effect." This failsafe image is not entirely unfounded, especially when it comes to traditional applications of DNA evidence. But increasingly DNA is being used for a new purpose: target the culprits in cold cases, where other investigative options have been exhausted. All told, U.S. law enforcement agencies have conducted more than 100,000 so-called cold-hit investigations using the federal DNA database and its state-level counterparts, which hold upward of 7.6 million offender profiles. In these instances, where the DNA is often incomplete or degraded and there are few other clues to go on, the reliability of DNA evidence plummets--a fact that jurors weighing such cases are almost never told. As a result, DNA, a tool renowned for exonerating the innocent, may actually be putting a growing number of them behind bars.

When police initially investigated Sylvester's murder in the early 1970s, the lead suspect was a man named Robert Baker, who just a month before the attack had escaped from a mental hospital and was living in a rundown Volkswagen bus near Fisherman's Wharf. Baker matched the description given by Sylvester's landlady, and two weeks before Sylvester's murder he had snuck into the apartment of another woman who lived just four blocks away and forced her to perform oral sex (a crime for which he was later convicted). In that case, as in Sylvester's, there was no sign of forced entry or a struggle. And while he hadn't killed the woman, he had threatened to do so, telling her, "I can rape you now or after you're dead." Days after Sylvester was killed, Baker also allegedly harassed a woman and a young girl, following them to their home just a few doors from Sylvester's apartment.

There was also other evidence linking Baker to the crime. When police searched his van, for instance, they found a blood-spattered parking ticket, and the blood type matched Sylvester's. And there...