CHICAGO (AP) _ Two defendants who were tried five times for the same killings were found innocent by a judge after a prosecutor from another county cast doubt on the credibility of the state's key witness.

''We looked in our computers, and we couldn't find any other defendants who have been tried five times in the same case,'' said William P. Murphy, attorney for Darby Williams, one of the two defendants.

Williams, 42, and Perry Cobb, 55, had spent nearly a decade in prison since the November 1977 killings of Melvin Kanter, 51, a Chicago hot-dog-stand owner, and his friend Charles Guccione, 67, during an apparent robbery.

The first two trials resulted in hung juries. The two men then were convicted and sentenced to death. But the Illinois Supreme Court reversed that decision and ordered a fourth trial, which resulted in another hung jury in 1986.

The innocent verdict came Tuesday after a bench trial presided over by Circuit Judge Thomas Hett. Murphy said he and Cobb's defense attorney chose a bench trial because Hett had indicated during a pretrial hearing that he had questions about the credibility of the state's key witness, Phyllis Santini.

The woman had testified earlier that she had done a favor for Cobb by keeping a car engine running while he and Williams went into the hot dog stand the day of the slayings.

But Michael Falconer, an assistant Lake County state's attorney, testified that he had worked with Ms. Santini at a factory before he attended law school and she had told him that a boyfriend of hers, not Cobb or Williams, had killed the two men.

Falconer, who did not appear at the first three trials, testified at the two final trials that he read about the case in a legal journal and remembered the woman's name.

''I can't help but believe him,'' Hett said of Falconer. The judge said Falconer was ''exhibiting the true job of a prosecutor - to do justice as he sees it.''