Republican Governor George Ryan announced a moratorium on executions on Monday, saying he wanted to know why more death sentences had been overturned in Illinois than carried out.
There have also been newspaper revelations that some death row inmates were defended by lawyers who were later suspended, and that other convictions were obtained through the questionable testimony of informants.
'Grave concerns'
Governor Ryan said: "I now favour a moratorium, because I have grave concerns about our state's shameful record of convicting innocent people and putting them on death row."
The 13 men freed were exonerated either through DNA evidence, witnesses who recanted or independent investigations.
In one high-profile case, a man spent 15 years on death row - once coming within two days of being executed - before a college journalism class proved his innocence.
"That's 13 mistakes, 13 mistakes that we know of," Ryan told a news conference. "I can't support a system which in its administration has proven to be so fraught with error that it has come so close to the ultimate nightmare: The state's taking of innocent life."
Governor Ryan's decision made Illinois the first of the 38 states with capital punishment to halt all executions while it reviews its death penalty procedures.
Investigating panel
He said he would halt all executions until a panel could report back to him about the status of the state justice system's handling of capital cases.
The panel's draft report last autumn called for a more thorough evidence discovery process and the creation of a "capital bar" of specially trained attorneys to represent defendants and prosecute death penalty cases.
Governor Ryan said he was convinced to act by statistics from a recent Chicago Tribune series on Illinois' handling of capital cases that showed 33 death row inmates were defended by attorneys who were later disbarred or suspended, and that 46 convictions were obtained through the testimony of jailhouse informants.
The newspaper series and the attention brought by the exonerations led the Illinois Supreme Court to appoint a 17-judge panel to examine ways to improve the system.
Death row statistics
According to statistics provided by the Death Penalty Information Centre, Illinois has 161 inmates on death row, ranking it eighth among the states.
It has executed 12 of the 610 people put to death in the United States since 1976, ranking it 13th, well behind the 206 executions in Texas.
Other states said to be considering legislation that would temporarily halt executions include Pennsylvania, Washington, New Jersey and Maryland