In 18 years with the Chicago Police Department, the nation’s second-largest, Jerome Finnigan had never been disciplined — although 68 citizen complaints had been lodged against him, including accusations that he used excessive force and regularly conducted illegal searches.

Then, in 2011, he admitted to robbing criminal suspects while serving in an elite police unit and ordering a hit on a fellow police officer he thought intended to turn him in. He was sentenced to 12 years in prison. “My bosses knew what I was doing out there, and it went on and on,” he said in court when he pleaded guilty. “And this wasn’t the exception to the rule. This was the rule.”

Mr. Finnigan is one of thousands of Chicago police officers who have been the subject of citizen complaints over the years but have not been disciplined by the department, according to data released this month by the Invisible Institute, a nonprofit journalism organization, and the Mandel Legal Aid Clinic of the University of Chicago Law School. Such information is rarely made public and has come to light in Chicago only after a decade-long legal battle by the institute and the clinic.

The trove of information — thousands of pages of officers’ names and brief descriptions of each civilian complaint against the Chicago police from March 2011 to September 2015 — provides a rare look into the cloistered world of internal police discipline.