Californian manhunt for ex-cop widens to Mexican border LOS ANGELES (Reuters) - U.S. Border Patrol agents on Saturday helped police widen the mountaintop manhunt for an ex-policeman wanted as a suspect in three California slayings, seeking to block his potential escape to neighboring Mexico. Former Los Angeles officer Christopher Dorner, 33, has declared war on law enforcement officers and their families in a manifesto posted on the Internet that complains of his 2008 firing from the Los Angeles Police Department (LAPD). Over the past several days, he is accused of having killed the daughter of a retired LAPD captain and the young woman's fiancé in Irvine, California, as well as a police officer from Riverside. Police have said they believe Dorner is carrying multiple weapons, including an assault-style rifle. The hunt for Dorner has centered on the area of Big Bear Lake, a popular ski resort in the San Bernardino Mountains about 80 miles northeast of Los Angeles, where his burning pickup truck was discovered on Thursday. Authorities have acknowledged he may have slipped away undetected. The axle of Dorner's truck was damaged, indicating he may have been forced to abandon it, said Irvine police spokesman Lieutenant Bill Whalen. There is speculation in the media, however, that Dorner may have left the truck as a diversion tactic. "At this point it was not determined when that damage occurred," Whalen said, adding it could have happened after the truck was abandoned and set on fire. On Saturday morning, authorities dispatched officers on the ground and air units to begin a third day of searches in the mountain, said Cindy Bachman, a spokeswoman for the San Bernardino County Sheriff's Department. Officers have used dogs and armored personnel carriers equipped with snow chains to maneuver through the mountains. Over 100 officers have been deployed to the area. The U.S. Border Patrol sent tactical response teams to the Big Bear Lake area to assist, said Border Patrol spokesman Steven Pitts. Those teams are typically heavily armed and specially trained. The U.S. Customs and Border Protection agency also sent a helicopter, Pitts said. "We did lend operational support to the San Bernardino County Sheriff in their search for this very dangerous man," he said. Federal agents at the border with Mexico are screening vehicles to prevent Dorner from fleeing the country, he said. Southbound traffic into the border town of Tijuana has been clogged, with the backup extending over 3 miles on Saturday. Dorner in his manifesto vowed to wage "unconventional and asymmetrical warfare to those in LAPD uniform whether on or off duty." But a military expert said such threats were overblown. "A guy can be a serial killer or sniper or just a murderer, he can be Jack the Ripper, but that's hardly guerrilla warfare," said Anthony James Joes, author of the book "Urban Guerilla Warfare." "I don't care how good this guy is, or how crazy this guy is, he's not going to surround anyone," said Joes, who teaches political science at Saint Joseph's University in Pennsylvania. Early on Thursday, he traded gunfire with two Los Angeles police officers who were tracking him, and later ambushed two Riverside officers. He killed one and left the other badly wounded, police said. Police said they were providing extra security for about 40 potential targets mentioned in Dorner's online declaration. It was posted to Facebook but has since been taken down. Dorner joined the Navy in 2002 and reached the rank of lieutenant. He received a rifle marksman ribbon and a pistol expert medal, according to his official service record. A Navy spokesman said those are basic-level commendations. As the mountaintop search continued, questions have arisen over how well prepared he might be to survive the cold winter conditions if he remains in the area. James Usera, an attorney in Salem, Oregon, played football with Dorner when both of them attended Southern Utah University in the late 1990s. He said Dorner, who grew up in urban southern California, did not have much outdoors experience then, but they would sometimes go jackrabbit hunting together in Utah. "My memory now is that I don't know that he ever had a gun in his hands" during those hunts, Usera said. Additional reporting by Marty Graham in San Diego and Brandon Lowrey in Los Angeles, Editing by Sandra Maler