CHICAGO (CBS/AP) — Anthony Porter, who spent more than 15 years on death row before his exoneration helped bring an end to the death penalty in Illinois, has died at age 66.

The Cook County Medical Examiner’s office said Porter died Monday night. An autopsy determined his death was the result of “anoxic brain injury, probable opioid toxicity,” and ruled the death an accident.

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Porter was convicted in the 1982 murders of Marilyn Green and Jerry Hillard, but was exonerated and released from prison after another man, Alstory Simon, confessed to the shootings during an investigation of the murders by a team of journalism students from Northwestern University. Simon was later convicted and sentenced to 37 years in prison.

Porter had once been only two days away from execution before his attorneys won him a stay after raising questions about his mental competence. Meanwhile, the state’s only witness implicating Porter in the crime recanted his story.

As CBS 2 reported at the time in February 1999, the mother of one of the victims also said she never believed Porter was the killer and said another man and his wife were responsible.

A team of journalism students at Northwestern and their professor later took up Porter’s case, and got Simon’s confession. Porter was exonerated in 1999.

But Simon’s attorneys later accused the Northwestern team of coercing him.

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The Cook County State’s Attorney’s Office re-examined Simon’s conviction in 2013 after he recanted his confession. Simon alleged he was coerced into making it by a private investigator, working with the journalism students, who he says promised him he would get an early release and a share of the profits from book and movie deals.

In 2014, State’s Attorney Anita Alvarez noted the investigation of the case was corrupted and her office could no longer maintain the legitimacy of Simon’s conviction. Alvarez would not say if she believed Simon was innocent.

Simon was released from prison in October, 2014.

Porter’s case helped lead former Gov. George Ryan to halt all executions in Illinois. Ryan declared a moratorium on executions in 2003 and cleared death row by commuting the death sentences of more than 150 inmates to life in prison. Illinois, led by Gov. Pat Quinn, abolished the death penalty in 2011.

Porter was arrested in 2011 for stealing deodorant from a Chicago pharmacy. He pleaded guilty to retail theft and was sentenced in 2012 to one year in prison.

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CBS 2 Chicago Staff