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About 2,500 people have lodged formal complaints in the last four and a half years with the New York Police Department, reporting that officers acted with bias toward them.

Not a single allegation has been substantiated by the Police Department.

In a report issued on Wednesday, a city watchdog agency found the investigations of these complaints against the police lacking, and recommended changes to how the department classifies and handles bias allegations.

Often, allegations were listed as unsubstantiated or unfounded, and closed, according to the report by the city’s Department of Investigation.

In some cases officers misclassified complaints or failed to interview people involved. The report also concluded that police officials did not take bias allegations as seriously as they do other kinds of police misconduct.