Such mistakes are called common. Florida leads the nation in reversals.
Citing a "shameful record of convicting innocent people and putting them on death row," Gov. George Ryan of Illinois on Monday halted all executions in the state, the first such moratorium in the nation.
Ryan, a moderate Republican who supports the death penalty but questions its administration, noted that 13 men had been sentenced to death in Illinois since 1977 for crimes they did not commit, before ultimately being exonerated and freed by the courts.
"I cannot support a system, which, in its administration, has proven so fraught with error," he said, "and has come so close to the ultimate nightmare, the state's taking of innocent life."
The announcement was hailed by opponents of the death penalty, who said mistaken convictions like those discovered in Illinois were common throughout the United States. Some 85 people have been found to be innocent and released from death row since 1973.
While the wrongful convictions in Illinois have generated intense scrutiny and debate, Florida has seen 18 death row cases reversed, the most of any state.
The Legislature in Nebraska passed a moratorium on executions last year, citing concerns of racial disparity in sentencing, but the governor vetoed it. Bills that would halt executions are pending in 12 states.
More than 600 inmates have been put to death since 1977, when the Supreme Court allowed the reinstatement of the death penalty. The death penalty is on the lawbooks in 38 states.
Ryan's announcement of a moratorium in Illinois, pending a review by a special panel, met with little public criticism in Illinois, even among ardent conservatives, a measure of how public outrage over the wrongful convictions has changed the political landscape on the issue in this state.
"Until I can be sure that everyone sentenced to death in Illinois is truly guilty, until I can be sure with moral certainty that no innocent man or woman is facing a lethal injection, no one will meet that fate," the governor said, according to the Associated Press.
One of the leading voices for a moratorium has been that of Mayor Richard M. Daley of Chicago, who, while he was Cook County state's attorney in the 1980s, prosecuted some of the death penalty cases that later were overturned.
Daley now contends that prosecutors did nothing improper in these cases but that defense lawyers were often poorly financed and sometimes incompetent.
In nine of the reversed Illinois cases, students and professors at Northwestern University unearthed pivotal evidence in the trials that freed the men from death row.
Lawrence Marshall, a law professor at Northwestern University who is director there of the Center for Wrongful Convictions, said that the mistakes unearthed in Illinois surely exist in other states.
"This should not be seen as an Illinois problem," said Marshall, who said defendants probably received fairer treatment in Illinois than in many other places. "What happened here is that we got lucky in the first few cases and found the evidence. After that, people were more willing to take a second look at other cases. It was a snowball effect."
While Illinois seems to have become a center of debate over the death penalty, the issue is gaining resonance around the nation, after many years in which it was seen as essentially a dead letter in American politics.
Besides the halt on executions here, and the vetoed moratorium in Nebraska, the death penalty has been the focus of intense scrutiny in Florida and other states.
Last year, in a visit to the United States, Pope John Paul II called on Catholics to oppose the death penalty. And a new movie, Hurricane, examines the life of a man, Reuben Carter, who was sent to death row for a crime he did not commit.