Law enforcement officials have lost the trail of Christopher Jordan Dorner, the fugitive ex-cop accused of killing three people in a revenge-fueled spree.

Authorities spotted fresh tracks in Big Bear Valley much earlier Thursday, leading away from a burned vehicle they believe was the fugitive's, and began combing the area looking for him.

But by late Thursday, the search had proved fruitless.

PHOTOS: Manhunt for ex-LAPD officer

Dorner, 33, is a former Los Angeles Police Department officer who allegedly shot and killed a Riverside police officer, the 28-year-old daughter of a former LAPD captain and her fiance, and wounded two other police officers. He is considered armed and dangerous, and a massive manhunt across California and into Nevada has ensued.

"They're looking for tracks. Tracks similar to the ones that led away from the burned vehicle," Cindy Bachman of the San Bernardino County Sheriff's Department said at a media briefing late Thursday.

"They have been searching now for many hours and they have not found" him, Bachman said. "It's very difficult to search in these conditions. The temperature, there's snow on the ground ... and it's dark, it's very, very dark."

TIMELINE: Manhunt for ex-LAPD officer

Bachman told Big Bear Valley residents not to answer their doors except for people they know or uniformed law enforcement. She said authorities were planning on searching some 400 homes and had already searched about half of them.

She said she was "not aware of any ties" Dorner has to the area.

Authorities said they had reduced the number of officers searching on foot Thursday evening and were relying heavily on aerial support. Nonetheless, they assured the search would continue through the night.

"It's been a tragic day for all law enforcement officers across the country," Bachman said of Dorner's alleged rampage and threats of continued violence.

"The person responsible for that is still on the street and we don't know what he's going to do," she said. "We know what he's capable of doing ... and we need to find him."

MORE:

Cop-killer is 'sick,' Villaraigosa says in touting gun control

Fugitive ex-cop stayed in San Diego hotel; military bases on alert

Manhunt: Ex-LAPD cop thought to have stalked victims, sources say

-- Joe Serna in Big Bear and Ari Bloomekatz in Los Angeles

Photo: Police officers check a house along Switzerland Avenue during the search Thursday for shooting suspect Christopher Dorner in Big Bear. Credit: Wally Skalij / Los Angeles Times

