The headline was disturbingly familiar: A person, usually male and often black, who has spent a substantial stretch of his life behind bars is freed after DNA evidence shows he is innocent.

That was the case for Horace Roberts, 60, who was released from a California prison 3 October after DNA evidence exonerated him in the 1998 killing of his former girlfriend and co-worker.

“I could not believe it was me walking out of prison,” Mr Roberts said when he was freed earlier this week. “When you are in prison, you do not know if you will ever get out.”

Mike Hestrin, the Riverside County district attorney, said in a statement: “What happened to Mr Roberts is tragic.”

He added, “Once I learned of the new DNA findings, I immediately directed that all charges be dismissed.”

In 1999, Mr Roberts was found guilty of second-degree murder and sentenced to 15 years to life in the killing of Terry Cheek, a co-worker he had supervised at Quest Diagnostics and with whom he had an affair.

He initially lied about that relationship to investigators after she was found strangled near a deserted stretch of Interstate 15 near Lake Corona, California, according to Michael Semanchik, the lead lawyer on Mr Roberts’ case and the managing lawyer at the California Innocence Project. Her body was discovered about a mile away from Mr Roberts’ truck, which she had often borrowed.

In the summer of 1997, Ms Cheek was still married to Googie Harris Sr when she began her affair with Mr Roberts, Mr Semanchik said.

By December, she had filed for divorce and was living between an apartment in Temecula, California, with Mr Roberts and a house in Riverside with Mr Harris. The divorce had become contentious, Mr Semanchik said. Mr Harris filed a restraining order against Ms Cheek and requested full custody of her two daughters.

One afternoon, Mr Roberts was waiting for Ms Cheek to pick him up with his truck to go to work. She never showed up. A few days later, her body was found.

When he was questioned by the police soon afterward, Mr Roberts denied the affair. The authorities showed him a photo of a watch found at the scene that they believed belonged to him. He agreed it appeared to be his. Later, his wife came forward to say she had found his watch at home.

He was convicted by a jury “based on his truck, lies he told at the police interview about their affair and the watch,” Mr Semanchik said.

After Mr Roberts had spent four years in prison, and after several denied appeals, a fellow inmate told him about the California Innocence Project. In 2003, Mr Roberts filled out and mailed a 75-page questionnaire to have his case reviewed. The non-profit group accepted the case.

The goal was to add Mr Roberts to the list of 362 people who have been exonerated by DNA evidence since Gary Dotson, the first such exoneree, was freed in 1989, according to data from the Innocence Project, a non-profit based in New York that is separate from the California Innocence Project.

Of those 362 cases, 130 exonerees were wrongfully convicted of murder. African-Americans make up nearly two-thirds of those exonerated by DNA, or 222 of the 362 former inmates.

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DNA testing of the watch and some rope found at the crime scene, as well as of debris found under Ms Cheek’s fingernails, turned up a DNA profile for an unidentified man and the DNA of Mr Harris’ son Googie Harris Jr, who was 19 at the time of the killing, according to Mr Semanchik.

Lawyers with the California Innocence Project presented their case to the district attorney’s office in 2012, but were unsuccessful at getting Mr Roberts exonerated.

The new DNA results connected the watch to Googie Harris Jr, but the unidentified man’s DNA was not sufficient to upload to the Combined DNA Index System, or CODIS — the FBI’s program that allows forensic laboratories to create and search databases of DNA profiles.

Mr Semanchik did not give up. Last year, his group devised another strategy: Test everything for DNA.

Mr Semanchik successfully petitioned for Ms Cheek’s bra, jeans, socks, shoes and fingernails to be retested for DNA. Now with enough DNA to upload to CODIS, the authorities matched it to a man named Joaquin Leal, 52, who turned out to be the elder Harris’ nephew.

“It appears Ms Cheek was fighting back and got some DNA on her right-hand fingernails,” Mr Semanchik said.

A background check on Leal showed that he had moved in with Mr Harris immediately after the killing and had been convicted of sexually assaulting the daughter of his uncle’s new girlfriend, according to Mr Semanchik.

Last week Leal and the elder Harris, 62, were arrested on suspicion of murder, with bail set at $1 million (£880,000) each. It was not clear at the time of reporting if they had legal representation. Prosecutors did not immediately respond to questions about the younger Harris’ involvement in the case.

Mr Roberts had been quietly released by the district attorney nine days before the arrests. He is eligible for the state to compensate him $140 for each day he was in prison, which would add up to just more than $1 million. If approved, he would receive the money in a lump sum.

“Hopefully he will find out in the next few months,” Mr Semanchik said.

When he was released, Mr Roberts knew exactly what he wanted to eat.

“I went and had me a Southern-style breakfast,” Mr Roberts said. “I had sausage, biscuits over gravy and hash browns,” he said. “I had not eaten like that since 1998, so it was really good.”