SPRINGFIELD, Ill. -- 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.