Al Sanchez was convicted at the high profile trial based on testimony from a key witness whose arrest record and gang affiliations should have been disclosed beforehand to defense attorneys but were not, Judge Robert W. Gettleman said.

FBI agents based in Indiana also should have told prosecutors in Chicago that the witness, Brian Gabriel, was under investigation in a gang war between the Spanish Vice Lords and Latin Kings at the time of the trial, Gettleman said.

And the prosecutors could have learned of that investigation if they had performed a records search, Gettleman said in his 22-page opinion.

"Based on these findings, this court has lost confidence in the integrity of the verdict convicting these defendants," Gettleman said in his 22-page opinion.

Gettleman also ordered a new trial for a youthful former aide to Sanchez, Aaron Delvalle, who was convicted of one count of perjury at the March trial.

The trial attracted the spotlight because as streets and sanitation commissioner Sanchez headed a department that for decades was a major pool of patronage jobs for the once mighty Chicago Democratic Machine. That spotlight was intensified because in recent years the U.S. attorney's office has conducted a major investigation of hiring fraud at City Hall.