SAN DIEGO – The devastating wildfires in Southern California have caused at least $1 billion in damage in San Diego County alone, officials said Wednesday, as easing wind gave firefighters hope that they could begin to gain ground against the flames.

Authorities were investigating arson as a possible cause of at least one of the wildfires. FBI evidence response teams recovered materials they hoped would identify the source of the fires that have burned for four days. The FBI said a house was not searched, correcting earlier reports from a law enforcement official.

Richard Kolko, an FBI spokesman in Washington, said the evidence response teams “have been working with other federal, state and local authorities” to identify the source of the fires.

The fires have destroyed 1,500 homes and caused at least a half-million people to flee – the largest evacuation in state history. At least 1,200 of the damaged homes were in San Diego County, and officials believe that number will rise.

“Clearly, this is going to be a $1 billion or more disaster,” Ron Lane, San Diego County’s director of emergency services, told reporters during a news conference.

The announcement of San Diego’s staggering losses came as President Bush signed a major disaster declaration for California in the wake of the wildfires that have charred about 426,000 acres, or about 665 square miles.

The declaration puts in motion long-term federal recovery programs to help state and local governments, families, individuals and certain nonprofit organizations recover. Bush plans to visit the state on Thursday.

“Americans all across this land care deeply about them,” the president said after a Cabinet meeting convened to coordinate federal relief efforts. “We’re concerned about their safety. We’re concerned about their property.”

The fierce Santa Ana wind that has stoked the explosive blazes had started to moderate Wednesday although stiff gusts continued to blow through some canyon areas. Forecasters said the wind eventually would be followed by cooling sea breezes.

Wind was reported blowing at a sustained speed of 21 to 36 mph in some areas Wednesday, considerably less than the gusts of up to 100 mph earlier in the week.

The shift could allow for a greater aerial assault and help firefighters beat back the most destructive blazes, said Homeland Security Secretary Michael Chertoff.

An unmanned NASA aircraft outfitted with high-tech imaging equipment took off Wednesday from Edwards Air Force Base on Wednesday for a 10-hour flight to help firefighters locate hot spots. Pilots at NASA Dryden Flight Research Center were remotely controlling the aircraft, outfitted with a thermal-infrared imaging system capable of seeing through thick smoke.

Crews also were anticipating additional firefighters and equipment from other states, mostly throughout the West. Frustration over the firefighting effort began to emerge Tuesday when a fire official said not enough had been done to protect homes.

Associated Press writers Matt Apuzzo, Chelsea J. Carter, Jeremiah Marquez, Daisy Nguyen, Robert Jablon and Thomas Watkins, Martha Mendoza, Jacob Adelman in Santa Clarita, Elliot Spagat, Allison Hoffman and Scott Lindlaw, Pauline Arrillaga, Ryan Pearson and Jennifer Loven in Washington contributed to this report