SAN DIEGO — The devastating wildfires in Southern California have caused at least $1 billion in damage in San Diego County alone, officials said yesterday, 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. Mr. Bush plans to visit the state today.

“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.”