LOS ANGELES (AP) - A fired police officer who threatened to bring "warfare" to the Los Angeles Police Department went on a shooting rampage that left a policeman and two others dead and set off an extraordinary manhunt that had three states and Mexico on alert for much of Thursday.

The search for Christopher Dorner led hair-trigger officers to mistakenly shoot at innocent citizens and forced police to guard their own.

But the focus of police efforts shifted Thursday afternoon to the snowy mountains around Big Bear Lake, about 80 miles east of Los Angeles, where police found Dorner's burned-out pickup truck and tracks leading away from the vehicle.

San Bernardino County Sheriff John McMahon said 125 officers were going door to door and attempting to track the suspect, and that a SWAT team was providing added security to those in the community. Schools were put on lockdown while investigators examined the vehicle and spread out across the area.

"He could be anywhere at this point, and that's why we're searching door to door," McMahon said, adding that the manhunt would continue "as long as we can." A snow storm was expected in the region with temperatures dipping into the teens overnight.

Said LAPD Assistant Chief Michel Moore: "This complex and violent investigation has led to this mountain."

The pickup was to be processed at a crime lab Thursday evening and examined by investigators from multiple agencies.

Throughout the day, thousands of heavily armed officers patrolled highways throughout Southern California, while some stood guard outside the homes of people police say Dorner vowed to attack in a rant posted online. Electronic billboards, which usually alert motorists about the commute, urged them to call 911 if they saw him.

"I will bring unconventional and asymmetrical warfare" to Los Angeles Police Department officers, on or off duty, said the manifesto. It also asserted: "Unfortunately, I will not be alive to see my name cleared. That's what this is about, my name. A man is nothing without his name."

Dorner, 33, had several weapons including an assault rifle, said police Chief Charlie Beck, who urged him to surrender at a news conference held amid heightened security in an underground room at police headquarters.

"Of course he knows what he's doing; we trained him. He was also a member of the Armed Forces," he said. "It is extremely worrisome and scary."

The nearly 10,000-member LAPD dispatched officers to protect more than 40 potential targets, including police officers and their families. The department also pulled officers from motorcycle duty, fearing they would make for easy targets.

"I never had the opportunity to have a family of my own, I'm terminating yours," the manifesto said.

At one point, officers guarding one location mistakenly opened fire on a pickup truck, believing it matched the description of Dorner's dark-colored 2005 Nissan Titan. Two occupants were injured.

The chief said there had been a "night of extreme tragedy in the Los Angeles area" and that the department was taking measures to ensure the safety of officers.

The search for Dorner, who was fired from the LAPD in 2008 for making false statements, began after he was linked to a weekend killing in which one of the victims was the daughter of a former police captain who had represented him during his disciplinary hearing. Thursday was the anniversary of his first day on the job at the department eight years ago.

Monica Quan and her fiance, Keith Lawrence, were found shot in their car at a parking structure at their condominium on Sunday in Irvine. Quan, 28, was an assistant women's basketball coach at Cal State Fullerton. Lawrence, 27, was a public safety officer at the University of Southern California.

Police said Dorner implicated himself in the couple's killings in the manifesto posted on Facebook. They believe he was the one who wrote it because there were details in it only he would know.

Dorner was with the LAPD from 2005 until 2008.