(CNN) -- Illinois put an end to capital punishment Wednesday as Gov. Pat Quinn signed legislation abolishing the practice and commuting the sentences of his state's remaining death row inmates.
The move came 11 years after a spate of death-row exonerations prompted one of Quinn's predecessors to halt executions.
In announcing his decision, Quinn said that a flawed system of capital punishment had put the state in "grave danger" of putting innocent prisoners to death and that creating a "mistake-free" system was impossible.
"Having said that, we cannot have a death-penalty system in our state which kills innocents, and unfortunately our system was in grave danger of doing exactly that in 20 instances," he said.
Illinois conducted its last execution in 1999. Then-Gov. George Ryan, a Republican, halted executions in 2000 after revelations that 20 death-row convicts were not guilty of the crimes that put them there. Ryan commuted 167 death sentences to life in prison without parole, and Quinn commuted another 15 on Wednesday.
Quinn's move leaves 34 states and the federal government with capital punishment on the books. Illinois is the third state to abolish the practice since 2007, and a court ruling struck down New York's death-penalty law in 2004.
Quinn, a Democrat, took office after the 2009 impeachment of Gov. Rod Blagojevich and won a full term in November. During the campaign, he said he would support keeping the death penalty for the "most severe" crimes. But he said Wednesday that he had "no regrets" about his decision.
"I think it's the right and just thing to abolish the death penalty and punish those who commit heinous crimes, evil people, with life in prison without parole or any chance of release," said Quinn.
Several prosecutors and the Fraternal Order of Police had lobbied to keep the death penalty, arguing that reforms passed after Ryan's moratorium had improved the system and that the threat of execution deterred crime.
Ted Street, the state FOP president, said execution should remain a potential punishment in "extreme cases" like the killing of a police officer or a child. And police and prosecutors have used the threat of the death penalty to help obtain a suspect's cooperation, he said.
"The criminal, when they know their cooperation in an investigation will yield that case not being forwarded to the death penalty, they might be more cooperative. And we've seen that many times," Street said. He predicted opponents in the legislature would try to reinstate capital punishment before long.
"I believe it's going to prove over time to be problematic," Street said.
And state Sen. William Haine, one of several Democrats to oppose repeal, said Quinn should have pushed for a statewide debate and a referendum on whether to keep capital punishment.
"This removes a remedy of the people of Illinois for great and evil acts of a unique kind: wanton cruelty, terrorism, rape and murder, the butchery of small children, mass murder," said Haine, from the southern city of Alton. "It removes a remedy for the community to seek the penalty of death in which someone forfeits one's life for these great wrongs committed to innocent people."
But Quinn's decision drew swift praise from the American Civil Liberties Union, which called it a "historic stand" by the governor and state lawmakers.
"Executions in this country are carried out as part of an unequal system of justice, in which innocent people are too often sentenced to death and decisions about who lives and who dies are largely dependent upon the skill of their attorneys, the race of their victim, their socioeconomic status and where the crime took place," John Holdridge, director of the ACLU Capital Punishment Project, said in a statement issued after Quinn's announcement.
"Such arbitrary and discriminatory administration of the death penalty, which comes at an enormous financial cost to taxpayers, is the very definition of a failed system, and the state of Illinois is to be commended for ending it."
And the Catholic Conference of Illinois said abolishing the death penalty "advances the development of a culture of life in our state."
"No longer will there be a risk in Illinois that an innocent person will be convicted and sentenced to death. The law becomes effective on July 1, 2011," the conference said in a written statement. It added, "Society will continue to be protected and those who commit crimes will still be held accountable through alternatives to the death penalty, including life without parole."