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They believe burly, heavily-armed Christopher Dorner is holed-up in the wilderness of California’s snow-capped San Bernardino mountains 80 miles east of Los Angeles.

The burnt-out shell of his pick-up truck was discovered in the nearby resort of Big Bear, where residents and tourists have been warned to stay indoors as the search continues.

Yesterday, as a task force of 125 officers, some riding Snowcats in the rugged terrain, continued their search, it was revealed that Dorner has become the first human target for remotely-controlled airborne drones on US soil.

A senior police source said: “The thermal imaging cameras the drones use may be our only hope of finding him. On the ground, it’s like looking for a needle in a haystack.”

Asked directly if drones have already been deployed, Riverside Police Chief Sergio Diaz, who is jointly leading the task force, said: “We are using all the tools at our disposal.”

The use of drones was later confirmed by Customs and Border Patrol spokesman Ralph DeSio, who revealed agents have been prepared for Dorner to make a dash for the Mexican border since his rampage began.

He said: “This agency has been at the forefront of domestic use of drones by law enforcement. That’s all I can say at the moment.”

Dorner, who was fired from the LAPD in 2008 for lying about a fellow officer he accused of misconduct, has vowed to wreak revenge by “killing officers and their families”.

In a chilling, 6,000 word “manifesto” on his Facebook page he has threatened to “bring warfare” to the LAPD and “utilise every bit of small arms training, demolition, ordinance and survival training I’ve been given.”

Dorner, 33, who rose to the rank of lieutenant in the US Navy and served in Iraq before joining the LAPD, also ominously warned that he has shoulder-launched surface-to-air missiles to “knock out” any helicopters used to pursue him.

Last night, Brian Levin, a psychologist and professor of criminal justice at Cal State University, San Bernardino, said: “We’re talking about someone who basically perceives that a tremendous injustice has been done to him that took his life and identity.

“Now he is, quite literally, at war.”

Dorner’s rampage began last Sunday when he shot dead Monica Quan, 27, the daughter of a former LAPD captain, and her fiancé Keith Lawrence as they sat in their car outside their home in Irvine, California.

Three days later, he stole a boat at gunpoint from an 81-year-old man at a yacht club in San Diego, near the Mexican border. He abandoned the boat when he could not get its engine to start.