ABC News' Pierre Thomas, Jack Cloherty and Jack Date report:

Armed with assault rifles and multiple other weapons, Christopher Dorner is at-large and hunting cops in California. A highly trained former Los Angeles police officer himself, Dorner has now become a police officer's worst nightmare.

"He knows what he's doing," L.A. Police Chief Charlie Beck told reporters today. "We trained him. He was also a member of the armed forces. It is extremely worrisome and scary, especially for the police officers involved."

In a "manifesto" attributed to Dorner posted on the internet, the former cop boasts about his combat skills, writing that he will utilize every bit of small arms training, demolition, ordnance and survival training he had been given in the LAPD and in his career in the Navy. Dorner's LAPD trainers rated him an "expert" sharpshooter with a 9 millimeter handgun, and a "marksman" with an M-16 rifle.

READ: Christopher Dorner Manhunt: Former LAPD Officer Has Cops Across California on Edge as Targets

Former FBI agent and ABC consultant Brad Garrett says Dorner's weapons training and his familiarity with police protocol make him an exceptionally lethal adversary.

"He knows how to move around and is very familiar with how to use firearms," Garrett said. "And knowing how the LAPD and other departments search for suspects, he will certainly avoid the logical places where police would look, like prior homes or locations that he used. He would also get rid of any electronics that might be used to track him," Garrett said. "He's probably using a pre-paid cellphone."

Garrett suspects that Dorner may have already ditched his truck, which had been identified by police, and picked up another vehicle. As a former Navy man, Dorner would still have his uniforms, and Garrett says he would not be surprised if he dressed in uniform to help avoid being spotted and identified.

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"A uniform has been part of his identity all of his life," Garrett says.

Dorner has already cut a wide path of destruction. Chief Charlie Beck said at a news conference today that Dorner "ambushed" two Riverside, Calif., officers as they sat in their squad car at a traffic light in the early morning hours today. One officer, a 35-year-old, 11-year veteran of the force, was killed. His 27-year-old partner was seriously wounded. Garrett says the ambush shows Dorner's skill as a human hunter.

"These officers apparently were taken by complete surprise. They didn't know they were being shot at until they were shot."

Dorner is also wanted in the killings of Monica Quan and her fiancé, Keith Lawrence, who were found shot to death in their car Sunday night, according to Irvine Police Chief David L. Maggard. Quan is the daughter of a retired LAPD official, and Dorner allegedly threatened that former official and his family, among others, in his manifesto, according to police.

"Other people besides law enforcement officers have been taken out," Garrett says. "It's clear that he thinks everyone is fair game. This is a dangerous and treacherous guy who for everyone's safety he has got to be taken off the street."

Dorner was fired from the LAPD in 2008 for making false statements about another officer's alleged brutality, and a second cop's alleged use of the "N" word. When Dorner's charges were found to be false by an LAPD internal review and subsequent appeals, he was fired from the force. In his "manifesto" titled "Last Resort," Dorner said he had "lost everything" because the LAPD suppressed the truth, and that would lead to deadly consequences.

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Now the manhunt for Dorner is in overdrive, and one source tells ABC News that just about anyone with a badge in Southern California is looking for him.

Local law enforcement agencies have activated "Code Alex," which triggers local police agencies to take up pre-planned observation posts as part of a mutual aid plan. And up and down the California coast, officers are being warned that they face a skilled and dangerous enemy.

Brad Garrett sees a violent end to it all. When asked what he thought the chances were of Dorner's surrendering to police, he said, "Below zero. Nothing about his personality indicates he would ever turn himself in. He will either be caught by surprise and taken, or caught in a shootout and go out in a blaze of glory."