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.

For example, the data for 2015 shows that in more than 99 percent of the thousands of misconduct complaints against Chicago police officers, there has been no discipline. From 2011 to 2015, 97 percent of more than 28,500 citizen complaints resulted in no officer being punished, according to the files.

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