By Dan Whitcomb

LOS ANGELES (Reuters) - Detectives in Los Angeles were following up on some 600 clues to the whereabouts of an ex-police officer suspected of killing three people, a Los Angeles Police Department spokesman said on Monday, the day after a $1 million reward was posted for information regarding the fugitive's whereabouts.

Christopher Dorner, 33, has not been spotted since early on Thursday, when he is accused of ambushing two policemen at a red light in Riverside, 60 miles east of Los Angeles. One of those officers was killed, the other wounded.

That confrontation came not long after the former Los Angeles police officer and U.S. Navy veteran is suspected of exchanging gunfire with police in nearby Corona, wounding one officer.

Aside from several false sightings, the search has largely focused on the snow-covered San Bernardino Mountains northeast of Los Angeles, where a truck belonging to Dorner was found burning on a fire road near the ski resort community of Big Bear Lake.

That effort has not been abandoned, LAPD spokesman Lieutenant Andrew Neiman said, but detectives were also busy following up hundreds of clues and tips from the public in hopes that one of them would lead them to the fugitive.

"As of 6 a.m. this morning, our investigators were working on over 600 clues, and I'm sure that number has increased since then," he said.

Dismissing speculation that Dorner's four-day silence may suggest he had taken his own life, Neiman said: "We are operating on the premise that he is still out and about and we are going to find him."

Police officials urged Southern Californians to call a tip line with any information on Dorner's whereabouts, but said that anyone who sees him should call 911 immediately instead.

ACTIONS 'CANNOT GO UNANSWERED'

Los Angeles Police Chief Charlie Beck has said that the $1 million reward, raised from private donations, police unions, businesses and local governments, marks the largest sum ever offered in Southern California in a criminal investigation.

Beck has also called the manhunt for Dorner the most extensive ever mounted in the Los Angeles area.

"This is a man who has targeted those who we entrust to protect the public. His actions cannot go unanswered," Beck told a news conference on Sunday.

A manifesto posted on Dorner's Facebook page last week claimed he was wrongly terminated from the LAPD in 2008 and vowed to seek revenge by unleashing "unconventional and asymmetrical warfare" on police officers and their families.

More than a week ago, he was named as a suspect in the slayings of a campus security officer and his fiance, the daughter of a retired Los Angeles police captain that Dorner's manifesto blames for his dismissal.

The couple, Keith Lawrence, 27, and Monica Quan, 28, were found shot dead eight days ago in their car on the top level of a parking structure in the city of Irvine, south of Los Angeles.

Dorner had ended his military service two days earlier, and the Navy has not disclosed the circumstances of his discharge.

Beck announced on Saturday a reopening of the inquiry into Dorner's firing to "reassure the public that their police department is transparent and fair."

The LAPD also has opened an investigation into an incident in which two women were wounded when officers opened fire on Thursday on a pickup truck resembling Dorner's in a case of mistaken identity. The two women, one of them aged 71, were delivering newspapers when they were shot.

(Reporting by Dan Whitcomb, Brandon Lowrey and Steve Gorman; Writing by Dan Whitcomb; Editing by David Gregorio)