Porter was convicted of killing a young couple on Chicago's South Side in 1982.
Now Alstory Simon, a Milwaukee laborer who used to live in Chicago, has admitted in a videotaped confession that he is the real murderer. He made his belated confession after seeing a report on CBS News This Morning about Porter's case.
In Milwaukee Wednesday, Simon said that the killings were in self-defense, over drug money.
"Before I knew anything, I just pulled it up and started shooting," he said in a taped confession. "I was thinking of trying to live. I had fear of my life."
After the shootings Simon fled Chicago with his wife Inez. She said she witnessed the killings and corroborates his story.
Last September, Porter was scheduled to be put to death. Just 48 hours before his execution, he was granted a temporary stay because his attorneys thought they could prove he was incompetent.
As CBS News Correspondent Cynthia Bowers reports, Porter's lawyers now believe they can prove he is innocent.
If Simon's confession holds up, it would be a remarkable twist in an investigation that began as a class project for students at Northwestern University.
Ciolino videotaped Simon's confession Wednesday morning after they viewed a television report on Porter's plight.
Ciolino told CBS This Morning Co-Anchor Thalia Assuras that "your story came out and he watched it. It confirmed everything I had been telling him earlier in our conversation. It seemed at that moment he gave up and he was ready to talk about the case and tell the truth."
"We sat him down, videotaped him," Ciolino continued, "and I have to tell you, the critical moment was when he watched the CBS story."
Because of these developments, Cook County prosecutors will now reopen the case.
If Porter is exonerated, he will be the 10th death row prisoner in Illinois released since the state reinstated capital punishment in 1977.