Illinois became the 16th state to ban capital punishment as Gov. Pat Quinn on Wednesday signed an abolition bill that the state legislature passed in January.

“Since our experience has shown that there is no way to design a perfect death penalty system, free from the numerous flaws that can lead to wrongful convictions or discriminatory treatment, I have concluded that the proper course of action is to abolish it,” Mr. Quinn said in a statement.

At a news conference at the Capitol in Springfield, Mr. Quinn said that signing the bill was the most difficult decision he had made as governor. “I have concluded, after looking at all the information that I have received, that it is impossible to create a perfect system  one that is free of all mistakes,” he said.

Mr. Quinn, a Democrat who became governor in 2009 and was elected to a full term in November, said during the 2010 campaign that he supported the death penalty when applied “carefully and fairly,” but added, “I am deeply concerned by the possibility of an innocent person being executed.” He had kept the question of whether he would sign the bill unanswered since it passed on Jan. 11.