LOS ANGELES — 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 Thursday that put Southern California on edge, led hair-trigger officers to mistakenly shoot at innocent citizens, and forced police to guard their own.
The search for Christopher Dorner had three states and Mexico on alert before shifting Thursday afternoon to the snowy mountains around Big Bear Lake, about 80 miles east of Los Angeles, where police found his 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 to track the suspect, and that a SWAT team was providing added security to 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 snowstorm 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 across Southern California, while some stood guard outside the homes of people police said 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 press conference held amid tight 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 easy targets.
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 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 Facebook manifesto. They believe he wrote it because there were details only he would know.
In the post, Dorner wrote that he knew he would be vilified by the LAPD and the news media, but that ‘‘unfortunately, this is a necessary evil that I do not enjoy but must partake and complete for substantial change to occur within the LAPD and reclaim my name.’’
Dorner was with the LAPD from 2005 until 2008.
According to documents from a court of appeals hearing, he was fired from the LAPD after he made a false complaint against his field training officer, Sergeant Teresa Evans. Dorner said that in the course of an arrest, Evans kicked suspect Christopher Gettler, a schizophrenic with severe dementia.
Dorner said in his online rant that after his dismissal that he lost everything, including his relationships with his mother, sister, and close friends.
‘‘Self-preservation is no longer important to me. I do not fear death as I died long ago,’’ the manifesto said. ‘‘I was told by my mother that sometimes bad things happen to good people. I refuse to accept that.’’