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The violence began with the weekend slayings of a campus safety officer and his fiancee, the daughter of a retired Los Angeles police captain who represented the accused gunman in disciplinary proceedings and who he blamed for his dismissal from the force.
Then, earlier this week, investigators learned that the suspect in that shooting, Christopher Dorner, 33, had posted an online declaration of grievances.
"This is a vendetta against all of Southern California law enforcement, and it should be seen as such," Los Angeles Police Chief Charlie Beck told reporters.
"He knows what he's doing. We trained him. ... He was also a member of the Armed Forces. It is extremely worrisome and scary, especially to the police officers involved," he added.
Police closed in Dorner at least once early on Thursday when two Los Angeles police officers exchanged gunfire with him in the city of Corona, leaving one of the officers grazed in the head by a bullet, police said.
Two other officers were ambushed, with one killed, about 20 minutes later while sitting in their patrol car at a traffic light in the adjacent town of Riverside, about 100km east of Los Angeles.
The officer who died was an 11-year veteran of the Riverside force. His partner was severely wounded but expected to fully recover, police said.
Dorner, who was fired from the Los Angeles Police Department in September 2008 after more than three years as an officer, also held the rank of lieutenant in the U.S. Navy Reserves, police and Navy officials said.
Beck said Dorner was presumed to be armed with multiple weapons, including an assault rifle. He said that threats contained in Dorner's rambling, multi-page Facebook manifesto had prompted police to dispatch more than 40 security details to protect people thought to be in danger of attack.
'HUNTING FOR POLICE OFFICERS'
At a separate news conference, Riverside Police Chief Sergio Diaz said he hoped Dorner could be captured without further bloodshed but warned that he was mobile and extremely dangerous.
"This is a somewhat unprecedented, or at least rare occurrence - a trained, heavily armed person who is hunting for police officers," he said.
The manifesto, quoted by various news media on Thursday, appeared to have been removed from the social network site, but Los Angeles television station KTLA posted a full copy of the document on its website.
"The violence of action will be high. ... I will bring unconventional and asymmetrical warfare to those in LAPD uniform whether on or off duty," Dorner wrote in the message. "The attacks will stop when the department states the truth about my innocence."
Dorner first came to the public's attention on Wednesday when he was named as a suspect in the weekend slayings of Monica Quan, an assistant basketball coach at California State University Fullerton, and her fiance, Keith Lawrence, a University of Southern California public safety officer.
Quan's father, retired LAPD Captain Randy Quan, had represented Dorner in disciplinary hearings that led to his termination from the department in September 2008 for making false statements, police said.
"I never had the opportunity to have a family of my own , I'm terminating yours," he wrote in a portion of his manifesto addressed to Randy Quan.
Dorner was said to be driving a dark gray pickup truck on Thursday, after what police believe was an aborted attempt Wednesday night to steal a boat from a yacht club in San Diego.
HIGHWAY PATROL ISSUES ALERT
He tied up the owner of a 45-foot vessel moored at the Southwestern Yacht Club, but was unable to get the boat's engine started and fled the scene, police Detective Gary Hassen said. The boat owner was unhurt.
The California Highway Patrol issued an alert on Dorner to law enforcement throughout the state after the Riverside shootings.
Los Angeles police placed officers on a tactical alert and grounded all motorcycle patrols as a precaution, an LAPD police spokeswoman said.
The manhunt also led to the wounding of two bystanders by police before dawn in Torrance, just south of Los Angeles, where officers on a security detail opened fire on a pickup truck resembling the one Dorner was thought to be driving, Beck said. He said the truck was being driven with its headlights out.
One victim suffered a minor gunshot wound and the second was listed in stable condition with two gunshot wounds.
"Tragically, we believe this was a case of mistaken identity by the Los Angeles police officers," Beck said.
The Los Angeles Times reported that the two victims were both women delivering newspapers.
Later in the morning, police in San Diego swarmed a residential facility near a military base as unconfirmed reports surfaced that a suspect had barricaded himself inside a building. Police said the incident turned out to be unrelated to the manhunt.