From the NY Daily News:

Cops talked to Elliot Rodger three times before Santa Barbara killing spree, didn’t know he owned guns. Rodger’s weapon ownership was available in law enforcement databases, which apparently were not checked when police investigated his strange behavior. He was armed with three legally purchased handguns and more than 400 rounds of ammo in his May 23 rampage in Isla Vista, Calif.

According to the article, the cops never bothered viewing the online videos posted by Roger — that had worried his mother — before visiting him a third time, just weeks before his rampage.