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BIG BEAR LAKE, Calif. (AP) -- More than 100 officers fanned out again at daybreak Saturday in the snow-covered San Bernardino Mountains, resuming the search for the former Los Angeles police officer suspected of going on a deadly rampage to get back at those he blamed for ending his career. Authorities hope clearer skies will allow aircraft to help them in the manhunt for Christopher Dorner, which entered its fourth day Saturday. Relentless snowfall on Friday grounded helicopters with heat-sensing technology and hampered their effort to find Dorner, whose burned-out pickup truck was found a day earlier in this ski resort town. SWAT teams in camouflage scoured the mountains and went door-to-door examining vacant cabins, aware to the reality they could be walking into a trap set by the well-trained former Navy reservist who knows their tactics and strategies as well as they do. "He can be behind every tree," said T. Gregory Hall, a retired tactical supervisor for a special emergency response team for the Pennsylvania State Police. "He can try to draw them into an ambush area where he backtracks." As authorities weathered heavy snow and freezing temperatures in the mountains, thousands of heavily armed police remained on the lookout throughout California, Nevada, Arizona and northern Mexico for a suspect bent on revenge and willing to die. Police said officers still were guarding more than 40 people mentioned as targets in a rant they said Dorner posted on Facebook. He vowed to use "every bit of small arms training, demolition, ordnance and survival training I've been given" to bring "warfare" to the LAPD and its families. The manhunt had Southern California residents on edge. Unconfirmed sightings were reported near Barstow, about 60 miles north of the mountain search, and in downtown Los Angeles. Some law enforcement officials said he appeared to be everywhere and nowhere, and speculated that he was trying to spread out their resources. For the time being, their focus was on the mountains 80 miles east of Los Angeles - a snowy wilderness, filled with thick forests and jagged peaks, that creates peril as much for Dorner as the officers hunting him. The small army hunting him has the advantage of strength in numbers and access to resources, such as special weapons, to bring him in. In his online rant, Dorner baited authorities. "Any threat assessments you generate will be useless," it read. "I have the strength and benefits of being unpredictable, unconventional, and unforgiving." Without the numbers that authorities have, Dorner holds one advantage: the element of surprise. Authorities said they do not know how long Dorner had been planning the rampage or why he drove to the San Bernardino Mountains. Property records show his mother owns undeveloped land nearby, but a search of the area found no sign of him. It was not clear if he had provisions, clothing or weapons stockpiled in the area. Even with training, days of cold and snow can be punishing. "Unless he is an expert in living in the California mountains in this time of year, he is going to be hurting," said former Navy SEAL Clint Sparks, who now works in tactical training and security. "Cold is a huge stress factor. ... Not everybody is survivor-man." Jamie Usera, an attorney in Salem, Ore., who befriended Dorner when they were students and football teammates at Southern Utah University, said he introduced him to the outdoors. Originally from Alaska, Usera said, he taught Dorner about hunting and other outdoor activities. "Of all the people I hung out with in college, he is the last guy I would have expected to be in this kind of situation," Usera, who had lost touch with Dorner is recent years, told the Los Angeles Times. Others saw Dorner differently. Court documents obtained by The Associated Press on Friday show an ex-girlfriend of Dorner's called him "severely emotionally and mentally disturbed" after the two split in 2006. Dorner served in the Navy, earning a rifle marksman ribbon and pistol expert medal. He was assigned to a naval undersea warfare unit and various aviation training units, according to military records. He took leave from the LAPD for a six-month deployment to Bahrain in 2006 and 2007. Last Friday was his last day with the Navy and also the day CNN's Anderson Cooper received a package that contained a note on it that read, in part, "I never lied." A coin riddled with bullet holes that former Chief William Bratton gave out as a souvenir was also in the package. Police said it was a sign of planning by Dorner before the killing began. On Sunday, police say Dorner shot and killed a couple in a parking garage at their condominium in Irvine. The woman was the daughter of a retired police captain who had represented Dorner in the disciplinary proceedings that led to his firing. Dorner wrote in his manifesto that he believed the retired captain had represented the interests of the department over his. Hours after authorities identified Dorner as a suspect in the double murder, police believe Dorner shot and grazed an LAPD officer in Corona and then used a rifle to ambush two Riverside police officers early Thursday, killing one and seriously wounding the other. The incident led police to believe he was armed with multiple weapons, including an assault-type rifle. That detail concerned officers whose bullet-proof vests can be penetrated by such high-powered weapons, said LAPD Deputy Chief Kirk Albanese. As a result, all LAPD officers have been required to work in pairs to ensure "a greater likelihood of coming out on top if there is an ambush," Albanese said. "We have no officers alone right now."