This investigation was published in partnership with The Marshall Project and FRONTLINE (PBS).

When the DNA results came back, even Lukis Anderson thought he might have committed the murder.

"I drink a lot," he remembers telling public defender Kelley Kulick as they sat in a plain interview room at the Santa Clara County, California, jail. Sometimes he blacked out, so it was possible he did something he didn't remember. "Maybe I did do it."

Kulick shushed him. If she was going to keep her new client off death row, he couldn't go around saying things like that. But she agreed. It looked bad.

Before he was charged with murder, Anderson was a 26-year-old homeless alcoholic with a long rap sheet who spent his days hustling for change in downtown San Jose. The murder victim, Raveesh Kumra, was a 66-year-old investor who lived in Monte Sereno, a Silicon Valley enclave 10 miles and many socioeconomic rungs away.

Around midnight on November 29, 2012, a group of men had broken into Kumra's 7,000-square-foot mansion. They found him watching CNN in the living room, tied him, blindfolded him, and gagged him with mustache-print duct tape. They found his companion, Harinder, asleep in an upstairs bedroom, hit her on the mouth, and tied her up next to Raveesh. Then they plundered the house for cash and jewelry.

After the men left, Harinder, still blindfolded, felt her way to a kitchen phone and called 911. Police arrived, then an ambulance. One of the paramedics declared Raveesh dead. The coroner would later conclude that he had been suffocated by the mustache tape.

Three and a half weeks later, the police arrested Anderson. His DNA had been found on Raveesh's fingernails. They believed the men struggled as Anderson tied up his victim. They charged him with murder. Kulick was appointed to his case.

Public defender Kelley Kulick was appointed to Lukis Anderson's case after he was charged with first-degree murder. Carlos Chavarría/The Marshall Project

As they looked at the DNA results, Anderson tried to make sense of a crime he had no memory of committing.

"Nah, nah, nah. I don't do things like that," he recalls telling her. "But maybe I did."

"Lukis, shut up," Kulick says she told him. "Let's just hit the pause button till we work through the evidence to really see what happened."

What happened, although months would pass before anyone figured it out, was that Lukis Anderson's DNA had found its way onto the fingernails of a dead man he had never even met.

Back in the 1980s, when DNA forensic analysis was still in its infancy, crime labs needed a speck of bodily fluid—usually blood, semen, or spit—to generate a genetic profile.

That changed in 1997, when Australian forensic scientist Roland van Oorschot stunned the criminal justice world with a nine-paragraph paper titled "DNA Fingerprints from Fingerprints." It revealed that DNA could be detected not just from bodily fluids but from traces left by a touch. Investigators across the globe began scouring crime scenes for anything—a doorknob, a countertop, a knife handle—that a perpetrator may have tainted with incriminating "touch" DNA.

Everyone, including Anderson, leaves a trail of DNA everywhere they go. Carlos Chavarría/The Marshall Project Sometimes that DNA can end up at a crime scene. Carlos Chavarría/The Marshall Project

But van Oorschot's paper also contained a vital observation: Some people's DNA appeared on things that they had never touched.

In the years since, van Oorschot's lab has been one of the few to investigate this phenomenon, dubbed "secondary transfer." What they have learned is that, once it's out in the world, DNA doesn't always stay put.

In one of his lab's experiments, for instance, volunteers sat at a table and shared a jug of juice. After 20 minutes of chatting and sipping, swabs were deployed on their hands, the chairs, the table, the jug, and the juice glasses, then tested for genetic material. Although the volunteers never touched each other, 50 percent wound up with another's DNA on their hand. A third of the glasses bore the DNA of volunteers who did not touch or drink from them.