The watchdog's findings said Durham Police failed to properly scrutinise Atherton's 2006 application for a shotgun certificate and an application for a firearms licence two years later in 2008.
To get the certificate, a buyer would need to be fingerprinted, take a firearms training course and undergo a national criminal background check and involuntary commitment or voluntary admission check.
Such a certificate would be issued after the buyer was fingerprinted, took a firearms training course and passed both a criminal background check and checks to see whether the person had been committed to a psychiatric hospital.