TMZ has learned ... alleged cop killer Christopher Dorner purchased a large scuba tank not for diving ... but to create his own air supply in case he was tear-gassed, which would allow him to ambush cops ... at least that is the main operating theory among law enforcement working on the case.
Although LAPD brass said at a news conference they were not in a position to confirm or deny Dorner is the person in the Feb 1st Sports Chalet surveillance video .... law enforcement sources connected to the case tell us they are "confident" Dorner is the man on the screen. The reason the brass will not say definitively if it's Dorner is because the scuba gear was a cash transaction, but we're told they know it's him.
Our sources tell us ... they believe Dorner did NOT fill the 2 small oxygen tanks and buy a large one for the purpose of diving. We're told they believe he knew he would end up being surrounded by cops while inside a building and they would probably use tear gas to subdue him.
We're told cops believe Dorner -- who is quite familiar with LAPD tactics -- may well have made the purchase to create his own oxygen supply, so he could withstand a tear gas attack and then ambush unsuspecting officers who went in to subdue him.