VENTURA, Calif. — The quick-moving fires that have forced thousands of Californians to evacuate their homes swept across the southern part of the state on Friday, destroying buildings and taxing fire crews that have been working for days.

Evacuees in San Diego County, where a 4,100-acre fire was only 15 percent contained as of late Friday night, told of swiftly encroaching flames that left at least six people injured, prized racehorses dead and dozens of homes in ruins.

“I got the ‘Get the hell out of here’ evacuation,” said Paul Anderson, who left his home in San Diego County on Thursday. “About four cop cars rolling around the neighborhood. ‘Get out!’ ”

The fire Mr. Anderson fled was among several burning Friday in Southern California, including a flare-up near Alpine in Cleveland National Forest and a blaze in Ventura County that has killed at least one person, burned 143,000 acres and destroyed more than 400 buildings — including at least 150 homes.

Officials said the Ventura fire was threatening homes from the exclusive enclave of Ojai to the hills of Ventura. Still, they said late Friday that conditions had improved, and they allowed many residents of the cities of Ventura and Santa Paula to return home.

President Trump on Friday declared a federal state of emergency in California and ordered additional aid. His declaration, which Gov. Jerry Brown had requested, allows federal agencies to coordinate the relief efforts.

On Friday night, the Ventura County Medical Examiner’s Office confirmed the first known, fire-related death this week. In a statement, the medical examiner said that the remains of Virginia Pesola, 70, of Santa Paula, were found at the site of a car crash on an evacuation route.

“This fire just keeps on going on us,” said Capt. Israel Pinzon, a spokesman for the state firefighting agency.