LOS ANGELES: Los Angeles police admitted Tuesday they were "frustrated," as a massive manhunt for fugitive alleged cop killer entered its sixth day without a result.

A $1 million reward offer has generated more than 1,000 tips about former LA Police Department (LAPD) officer Christopher Dorner, including a video showing him buying scuba gear shortly before going on the run, they said.

But although there have been reported sightings, police still appear little closer to finding the 33-year-old, who they admit may have fled across the border from California into neighboring Mexico.

"It is frustrating. We are hoping something will break loose," LAPD spokesman Andy Neiman told reporters, saying the number of tips received from the public had jumped from 250 to over 1,000 since the reward offer.

He confirmed that police were studying a store security video said to show Dorner buying scuba equipment days before he went on the run.

In the video, obtained by celebrity news website TMZ, a man who looks like Dorner is seen getting refill oxygen tanks used by scuba divers, in a Sports Chalet store in Torrance, south of Los Angeles.

Police have not confirmed the man is Dorner, but the LA Times cited sources saying it was the fugitive former cop.

Dorner, who posted a chilling online manifesto threatening to kill policemen and their families over his 2008 sacking, was charged Monday over the killing of officer Michael Crain, a 34-year-old father of two.

The six-foot (1.80 meters) tall naval reservist is accused of killing a couple on February 3, then ambushing two policemen last Thursday, including Crain. He also injured another officer, before going on the run.

The manhunt initially centered on the snowy mountains around Big Bear, two hours east of Los Angeles, after his burnt-out car was found there. Authorities are still searching in the mountains there, although the manhunt is much wider.