Christopher Dorner, the embittered former US cop on the run since last week, was charged Monday with murder in California, as authorities said he may have fled south into Mexico.
The 33-year-old, who posted a chilling online manifesto threatening to kill policemen and their families over his 2008 sacking, was charged over the killing of officer Michael Crain, a 34-year-old father of two.
Dorner was also charged with three counts of attempted murder, for allegedly shooting at three other officers, said Paul Zellerbach, district attorney for Riverside County, east of Los Angeles.
"The highest level of resources will continue to be expended in the effort to apprehend this fugitive," Zellerbach told reporters, adding: "We will bring him to justice. We're asking for the public's assistance finding him."
Los Angeles Mayor Antonio Villaraigosa announced that all city flags will be flown at half-staff in honor of Crain, whose funeral will be held on Wednesday.
The LA Police Department (LAPD) and other law enforcement agencies in southern California have tightened security around their facilities since Dorner went on the run last week.
Dorner, a six-foot (1.80 meters) tall naval reservist, is accused of killing a couple on February 3, then ambushing two policemen last Thursday, including Crain. He also injured another officer, before going on the run.
The manhunt initially centered on the snowy mountains around Big Bear, two hours east of Los Angeles, after his burnt-out car was found there. But after four days, there appears little likelihood he will be found at the mountain resort.
The disgruntled ex cop threatened to murder police and their families in a rambling manifesto posted online last week, pledging to "bring unconventional and asymmetrical warfare to those in LAPD uniform whether on or off duty."
Some 50 LAPD officers and families have been placed under special protection. The couple murdered two weeks ago included the daughter of a former LAPD officer linked to the fugitive ex-cop's dismissal in 2008.
On Saturday police announced they would open a new probe into Dorner's firing from the LAPD. On Sunday, Villaraigosa announced a $1 million reward for information leading to Dorner's capture.
Then on Monday the Los Angeles Times published documents filed in federal court suggesting that Dorner may have fled south into Mexico.
"There is probable cause to believe that Dorner has moved and traveled ... from California to Mexico with the intent to avoid prosecution," said an affidavit.
The documents listed the allegations against Dorner, and cited "recent observations of a suspect matching DORNER's description attempting to flee to Mexico."
DA Zellerback said Dorner may just be lying low, and planning to strike again. "Even though he may have gone underground now, given the nature of his conduct and his words and his actions, he's going to reappear," he said.
"I don't think he's done. Just look at his manifesto, and look at his actions. He's trying to send a message, and it would be my belief that his message is not completed yet."