Illinois State Police have admitted wrongly issuing a gun license to Gary Martin, the man who officials say shot five people dead in his former workplace in Aurora on Friday before being killed by cops. In a statement issued Monday, the state police acknowledged a screening process failed to detect Martin’s 1990s felony conviction for aggravated assault in Mississippi for which he served five years in prison, the Chicago Tribune reports. He received a gun license on Jan. 31, 2014, and purchased a .40-caliber Smith & Wesson from an Aurora gun dealer five weeks later—that was the gun used in the deadly attack. The agency said it revoked Martin’s license on April 17, 2014, and suggested the Aurora Police Department may have failed to do its part to ensure the man no longer owned a weapon. State police said the agency is reviewing Illinois, Mississippi, and federal records to determine how the conviction slipped through two criminal background checks.