SPRINGFIELD, Ill. (AP) — After two decades of debate about the risk of executing an innocent person, Illinois abolished the death penalty Wednesday, a decision that was certain to fuel renewed calls for other states to do the same.

Gov. Pat Quinn, a Democrat who has long supported capital punishment, looked drained moments after signing the historic legislation. Lawmakers sent him the measure back in January, but Quinn went through two months of intense personal deliberation before acting. He called it the most difficult decision he has made as governor.

"If the system can't be guaranteed, 100-percent error-free, then we shouldn't have the system," Quinn said. "It cannot stand."

Illinois becomes the 16th state in the nation without a death penalty more than a decade after former Gov. George Ryan imposed a moratorium on executions out of fear that the justice system could make a deadly mistake.

Quinn also commuted the sentences of all 15 men remaining on death row. They will now serve life in prison with no hope of parole.

In his comments, the governor returned often to the fact that 20 people sent to death row had seen their cases overturned after evidence surfaced that they were innocent or had been convicted improperly.

Death penalty opponents hailed Illinois' decision and predicted it would influence other states.

"This is a domino in one sense, but it's a significant one," said Mike Farrell, the former "MASH" star who is now president of Death Penalty Focus in California.

The executive director of a national group that studies capital punishment said Illinois' move carries more weight than states that halted executions but had not used the death penalty in many years.

"Illinois stands out because it was a state that used it, reconsidered it and now rejected it," said Richard Dieter of the Death Penalty Information Center in Washington.

New Jersey eliminated its death penalty in 2007. New Mexico followed in 2009, although new Republican Gov. Susana Martinez wants to reinstate the death penalty.

In New York, a court declared the state's law unconstitutional in 2004.

Quinn's decision incensed many prosecutors and relatives of crime victims. Robert Berlin, the state's attorney in DuPage County, west of Chicago, called it a "victory for murderers."

The governor reflected on the issue week after week, speaking with prosecutors, crime victims' families, death penalty opponents and religious leaders. He consulted retired Anglican Archbishop Desmond Tutu of South Africa and met with Sister Helen Prejean, the inspiration for the movie "Dead Man Walking.

A Chicago woman whose teenage son was gunned down in 2006 said the killer, who has never been caught, should not be allowed to breathe the same air she breathes.

"I am a Christian. I never believed in killing nobody else," Pam Bosley said, explaining her change of heart after her son was shot outside a church. "But the pain you suffer every single day, I say take them out."

Quinn said capital punishment was too arbitrary. A prosecutor in one county might seek the death penalty, while another prosecutor dealing with a similar crime might not, he said. And death sentences might be imposed on minorities and poor people more often than on wealthy, white defendants.

A Gallup poll in October found that 64 percent of Americans favored the death penalty for someone convicted of murder, while 30 percent opposed it. The poll's margin of error was plus or minus 3 percentage points.

The high point of death penalty support, according to Gallup, was in 1994, when 80 percent were in favor.

Doubts about Illinois' death penalty grew steadily throughout the 1990s with each revelation of a person wrongly sentenced to die — people like Anthony Porter.

Porter had ordered his last meal and even been fitted for burial clothes when, just 48 hours before his execution, lawyers won a stay to study the question of whether he was mentally capable of killing. That provided time for a group of Northwestern University students to gather information proving Porter's innocence.

Illinois was also the place where Ryan called for clemency hearings for all death row inmates — proceedings that involved a parade of people describing in heartbreaking detail how their children, parents, siblings and spouses died by violence.

Ultimately, Ryan told his staff, "I can't play God," and he cleared death row in 2003 by commuting 167 death sentences to life in prison and pardoning four people.

That delivered a jolt to the death penalty debate that was felt around the world.

A few years earlier, the Republican governor had halted all executions, and his Democratic successors continued the moratorium. Illinois' last execution was in 1999.

On Wednesday, Republican lawmakers immediately began discussing legislation for a new, narrower death penalty. They said safeguards added to the system after Ryan cleared death row — protections negotiated in part by President Barack Obama when he was a state senator — had eliminated any real danger of executing an innocent person.

Republican Rep. Jim Durkin of Westchester predicted Quinn will pay a political price if he seeks re-election in four years. Some terrible murder that cries out for the death penalty is bound to occur and grab voters' attention, he said.

Quinn said he would oppose any attempt to reinstate a new version of the death penalty. He also promised to commute the sentence of anyone who might receive a death sentence between now and when the measure takes effect on July 1, a spokeswoman said.

The governor sought to console those whose loved ones had been slain, saying the "family of Illinois" was with them. He said he understands victims will never be healed.

Bill Sloop, a truck driver from Carthage, said he was saddened to think that taxpayers would have to continue feeding, clothing and caring for Daniel Ramsey, the death row prisoner who killed his 12-year-old daughter and wounded her older sister in a 1996 shooting spree.

Quinn "shouldn't have done what he did," Sloop said.

Illinois Attorney General Lisa Madigan appealed directly to Quinn to veto the bill. Quinn's lieutenant governor, Sheila Simon, herself a former prosecutor, urged him to sign it.

Illinois has executed 12 men since 1977, when the death penalty was reinstated. The last person put to death was Andrew Kokoraleis on March 17, 1999. At the time, the average length of stay on death row was 13 years.

Kokoraleis, convicted of murdering and mutilating a 21-year-old woman, died by lethal injection.

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Associated Press writers John O'Connor and Zachary Colman in Springfield and Deanna Bellandi, Don Babwin and Karen Hawkins in Chicago contributed to this report.