Local, state and federal law enforcement agencies are focusing on the forested mountains around the California ski resort of Big Bear, in their frantic search for a disgruntled former police officer believed to be responsible for a series of fatal shootings.

Christopher Jordan Dorner, 33, is suspected of killing three people in Southern California this week. The truck he is thought to have been driving was found abandoned and burning on a forest service road on Thursday afternoon, around 100 miles from Los Angeles.

The manhunt began after Dorner posted a rambling online manifesto threatening police officers and their families with “unconventional and asymmetrical warfare”. The former US Navy reservist was fired from the LAPD in 2009 for lodging a false complaint against his training officer.

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Among those named in the manifesto was Randy Quan, a retired LAPD captain who was involved in Dorner’s dismissal. On Sunday, Quan’s daughter Monica and her fiancé were shot and killed in Orange County. In the early hours of Thursday, Dorner is suspected of shooting three police officers two separate incidents in Riverside County. One died from his injuries, another was seriously wounded and undergoing surgery.

On Wednesday evening, the fugitive is thought to have checked in to a motel used by naval officers and their families near San Diego, and to have tried to steal a 47-foot boat from an 81-year-old man at a nearby yacht club, in a bid to flee to Mexico. He was thwarted when a rope lodged in the boat’s propeller; the elderly owner escaped unharmed.

Dorner is said by the authorities to be heavily armed. At a press conference, Los Angeles police Chief Charlie Beck described the situation as “extremely worrisome and scary”. Officers assigned to protect another police official whom Dorner had threatened shot two women on Thursday morning. One of the women – who had been delivering newspapers in Torrance, south of Los Angeles – was shot in the hand, the other in the back. Both were rushed to hospital. They had been driving a blue pick-up resembling Dorner’s 2005 Nissan Titan.

A second shooting incident in Torrance also involved officers who came across a vehicle they believed to be Dorner’s. No one was hurt in that instance, but local television news stations recommended that owners of dark pick-up trucks should use alternative transportation until the he was apprehended.