The extraordinary conversion of a former US state governor from being a supporter of the death penalty to a fervent anti-death penalty campaigner has been made into a documentary film.
Mr Ryan was criticised by victims' families but hailed by rights groups
The film, Deadline, shows how Illinois's Republican Governor George Ryan - strongly pro-death penalty when elected in 1998 - first began to have serious doubts about its use.
He went on to set free four men he was convinced had been victims of a miscarriage of justice, and two days before he left office in 2003 he granted a blanket clemency, commuting the death sentences of 163 men and 4 women.
Mr Ryan told BBC World Service's Outlook programme that he had first begun to have doubts about the death penalty when Illinois released Anthony Porter, who had spent 16 years on death row for a crime he did not commit.
"He was released not by the system, but by a group of journalism students from Northwestern University, who went out and went through his case, found the real killer and got a taped confession from him.
"Anthony Porter was released from prison. I watched that on television."
Deadline was recently broadcast by the US network NBC after a representative saw it at the Sundance Film Festival.
The documentary features footage of the special clemency hearings, as well as interviews with the death row prisoners, exonerated men and Governor Ryan himself.
Wrongful convictions - such as that of Gary Dotson - helped change Mr Ryan's mind
Deadline shows how evidence began to accumulate, showing a serious risk of people being executed on shaky evidence.
The Chicago Tribune newspaper, meanwhile, also conducted an investigation which found that a number of convicted prisoners on Death Row had been tried under questionable circumstances.
This investigation had a profound effect on Mr Ryan, who highlighted some of its findings.
"I always thought it was a pretty meticulous system, that worked well... until I found out how bad and how broken the system really is, and still is," he said.
He said that before being governor he had not studied much about the death penalty.
But the responsibility of having to sign the papers ordering executions led him to think harder about his role.
Chicago Tribune study
33 African-American men on death row in Illinois tried by all-white juries
45 sentenced to death had been represented by attorneys who had been disbarred or suspended
Some convicted by single eyewitnesses testimony
"By being made governor, it made me executioner for the state. I had final say about who was executed and who wasn't," he said.
"Before anybody goes to the electric chair, the date comes from the Supreme Court and the governor signs off and they're executed - or he doesn't sign off and they live.
"It's an awesome responsibility."
At the time, Mr Ryan's decision was met with outrage and dismay by prosecutors and relatives of murder victims.
One victim's father, Vern Fuling, described it as a "mockery."
But Mr Ryan said that the more cases he looked at, the more he found there were "a lot of errors of in the system, and a lot of chance for error."
"That's when I called the moratorium and stopped the whole machinery of death in Illinois."