An ordinance requires all Chicago residents who have been convicted of specific gun-related offenses to register with the Chicago Police Department.
But an inquiry into its implementation and enforcement found the registry is incomplete and unreliable.
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The report says that's due in part to CPD's "lack of proactive enforcement of the ordinance."
The OIG is calling on the police department to invest the time and resources to make it effective in keeping Chicagoans safer.
"As Chicago reckons with the scourge of gun violence, we ought to be measuring up every available tool to control it," said Deborah Witzburg, inspector general for the city of Chicago.