600 tips as hunt for fugitive ex-cop, Christopher Dorner, continues (UPDATES)

Investigators have received more than 600 tip-offs on the whereabouts of fugitive ex-cop Christopher Dorner since posting a $1million reward for his capture.

Dorner is suspected of targeting police officers and their families in three killings committed in revenge for being fired in 2008.

Police said they were prioritising the leads as day four of the search for Dorner drew to a close, Los Angeles Police Department Lt. Andy Neiman said, the Los Angeles Times reported.

Neiman told media the search at Big Bear, although scaled back, remains a "critical piece of the investigation," saying authorities would remain on the mountain "until we've looked in every nook and cranny."

Border patrols in Nevada and California, and staff at airports were also put on alert.

Dorner was formally charged Monday with murdering a Riverside police officer, a charge that could bring the death penalty if convicted, ABC News reported.

Police were hopeful that the hundreds of tips could produce a credible lead. As part of their investigation, they surrounded and evacuated a Lowe's Home Improvement store in LA's San Fernando Valley Monday but found no evidence of Dorner, police spokesman Gus Villanueva said.

"This is a man who has targeted those who we entrust to protect the public. His actions cannot go unanswered," Los Angeles Police Department Chief Charlie Beck said, Reuters reported.

Authorities have also been working to protect dozens of officers and families who are possible targets based, CBS News reported.

Dorner released a 6,000-word "manifesto" on his Facebook page which made threats against individuals he believed were responsible for ending his career with the police force five years ago. 

"Our dedication to catch this killer remains steadfast. Our confidence remains unshaken," Los Angeles Mayor Antonio Villaraigosa said at a news conference.

"We will not tolerate this reign of terror."

Dorner has been on the run since Thursday when police found his pickup truck engulfed in flames near Bear Mountain after shooting three police officers, killing one and injuring the other two.

The search in southern California on the weekend centered around Big Bear after fresh tracks were found, but heavy snowfall hampered their efforts, ABC News reported.

The LA Times reported there have been many reported sightings of the 33-year-old, including false alarms on an Amtrak train near downtown Los Angeles as well as the Twin Towers jail on Friday. 

Law enforcement officials say Dorner appears to be everywhere and nowhere, and speculate that he's trying to spread out their resources.

John Miller, a former head of the LAPD's Major Crimes Division, told CBS News Thursday that authorities were carrying out a manhunt "like none other" to find him.

Dorner is believed to be connected to two murders in Orange County as well, and authored an online manifesto threatening to harm police officials and their families, according to the Los Angeles Times.

"I will bring unconventional and asymmetrical warfare to those in LAPD uniform whether on or off duty," said Dorner's manifesto, according to the Associated Press. 

He is reportedly carrying an assault rifle and multiple other weapons, according to a tweet from the AP: 

He was terminated from the Los Angeles Police Department in 2008 after six years with the force for making false statements, the San Jose Mercury News said.

The Los Angeles Police Department said in a written statement that it was cooperating and that Dorner "has made threats against members of the LAPD, and we are taking those threats very seriously," adding that the department was protecting those that had been threatened, USA Today reported.

"Dorner is to be considered armed and extremely dangerous and we ask that anyone who sees Dorner, to not approach or attempt contacting him, but to immediately call 911 and notify law enforcement authorities,'' the statement added.

The LAPD, a force 10,000-members strong, sent many officers to protect possible targets, and pulled cops from motorcycle duty, worried that they would be too easily targeted, the AP reported. 

The ex-cop is believed to be responsible for the deaths of Monica Quan and her fiance, Keith Lawrence, who were shot dead in their car in a parking structure in Fullerton, California over the weekend, the Mercury News reported.

28-year-old Quan's father represented Dorner during a disciplinary board hearing that ended in him losing his job with the LAPD, the Associated Press reported. 

The suspect is also believed to have wounded two women in Torrance.

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