Investigators suspect Christopher Dorner of killing a police officer and another officer's daughter and her fiance in retaliation for Dorner's firing from the LAPD in 2008. Two other people were injured.
Police found Dorner's torched pickup truck Thursday near Big Bear Lake in a mountainous area about 100 miles east of the Los Angeles area where the killings took place.
Police said if Dorner hid in the mountains an approaching snowstorm could make the search more difficult. Early Friday, the National Weather Service issued a winter storm warning for the area, with up to 10 inches of snow possible and wind gusts of up to 50 mph.
Authorities warned residents of mountain communities near San Bernardino, Calif., to stay indoors as police hunted for the former police officer.
"Dorner is considered to be armed and extremely dangerous," Los Angeles Police Chief Charlie Beck said.
"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," Beck said.
Police sent protection teams to guard uniformed officers and their families overnight. Motorcycle police were ordered off the roads and told to patrol only in squad cars.
In Torrance, near Long Beach, two women delivering newspapers were shot and wounded by police who mistook their Honda pickup for the gray Nissan identified as belonging to Dorner, police said.
Dorner, 33, who worked for the Police Department from 2005 to 2008, posted a 6,000-word threatening note on his Facebook page, which police referred to as his "manifesto."
He complained of severe depression and vowed to kill officers to avenge his dismissal for filing a false report accusing a colleague of abuse.
"I have exhausted all available means at obtaining my name back. I have attempted all legal court efforts within appeals at the Superior Courts and California Appellate courts. This is my last resort," Dorner wrote.
"The LAPD has suppressed the truth and it has now [led] to deadly consequences. I will bring unconventional and asymmetrical warfare to those in LAPD uniform whether on or off duty."
Besides posting his manifesto, Dorner reached out to the media, mailing a parcel to CNN anchor Anderson Cooper's office New York.
The package arrived Feb. 1 and was opened by Cooper's assistant. Inside was a hand-labeled DVD and a yellow sticky note reading, in part, "I never lied" -- apparently referencing his 2008 dismissal.
The focus of the search shifted for a time to the San Diego area, where a caller told authorities Dorner may have been spotted near the Barona Indian Reservation. Sgt. Jason Rothlein of the San Diego County Sheriff's Department later said the tip was probably a hoax.