Christopher Dorner dragnet is extended to Mexican Border

(ABC NEWS) -- The dragnet for suspected cop killer Christopher Dorner has been extended as far south as the Mexican border as the Border Patrol is scrutinizing more carefully people trying to cross into Mexico.

The focus on the border, which is creating longer lines than normal at the San Diego-Tijuana border crossing, comes as law enforcement continue to comb through Big Bear Mountain where Dorner's burned truck was found last week, but that effort appears to be scaling back.

The dragnet was expanded as Dorner, a dismissed Los Angelese Police Department cop, was formally charged with murdering a Riverside police officer, a charge that could bring the death penalty if Dorner is captured and convicted.

The charges do not involve two other people Dorner is suspected of killing in Orange County.

The Border Patrol said they were heightening their checks of vehicles and pedestrians passing south into Mexico.

"Customs and Border Protection officers and agents from the Office of Field Operations, the U.S. Border Patrol and the Office of Air and Marine are providing assistance in the search as requested from law enforcement authorities," a statement from the Border Patrol said. An official with the agency said the request was part of the search for Dormer.

The statement said that because of the extra precautions, "travelers heading southbound into Mexico may experience delays."

Dorner, 33, is suspected of killing Riverside police officer Michael Crain and seriously wounding another in an ambush that occurred last Wednesday. He is also suspected of killing Monica Quan, the daughter of ex-Los Angeles Police Department captain Randal Quan, and her fiancé Keith Lawrence, who were found shot to death on Feb. 3 in the parking lot of their condominium complex in Irvine, Calif.

"By both his words and conduct, he has made very clear to us that every law enforcement officer in Southern California is in danger of being shot and killed," Riverside District Attorney Paul Zellerbach said.

The manhunt was in its fifth day Monday in the San Bernardino Mountains near the popular ski resort town of Big Bear, roughly 80 miles northeast from Los Angeles.

A $1 million reward for the capture and conviction of Dorner has fostered hundreds of clues on Monday that authorities are investigating in an attempt to capture Dorner.

Within a few hours of the reward announcement, a reported Dorner sighting led to an evacuation of a Lowe's Home Improvement store on Sunday in Los Angele's San Fernando Valley. Helicopters hovered overhead and a command center was established, however after searching the store, authorities were unable to find any evidence that Dorner was there or had been there.

The last clue to Dorner's whereabouts was when his burned out truck was found last Thursday on the side of Big Bear Mountain which contained weapons and had a broken axle. The search was scaled back over the weekend and a helicopter equipped with heat-seeking technology is assisting two-dozen officers by scanning the area while they return to some of the 600 cabins they earlier visited.

Though he remains at large, authorities believe that Big Bear remains his most likely location and planning may have helped him avoid capture thus far.

Dorner released a 6,000 word so-called "manifesto" on his Facebook page which outlined his distaste for the Los Angeles Police Department and made threats against individuals he believed were responsible for ending his career with the police force five years ago. The LAPD has assigned 50 protection details to guard officers and their families who were deemed possible targets.

Despite the costs, there are no plans to reduce the protection until Dorner is in custody, Los Angeles police Sgt. Rudy Lopez said.

Between the manhunt, protection details, and the need for additional security at Sunday's Grammy Awards, the Los Angeles Police Department enacted a tactical alert Sunday afternoon that would remain in effect through Monday; the alert can require officers to stay on duty beyond their shifts and asks them not to respond to low-priority radio calls.

ABC News' Alyssa Newcomb and The Associated Press contributed to this report

DANIEL CLARK and ALEX STONE, ABC News