LOS ANGELES — The fire that burned through one of the wealthiest neighborhoods in the country last week started as a cooking fire at a homeless encampment, according to an investigation by the Los Angeles Fire Department.

Homeless people had been living in the neighborhood — Bel-Air, in northwest Los Angeles — making their encampment near an underpass of the 405 freeway along Sepulveda Boulevard for several years, said Peter Sanders, a spokesman for the fire department. The blaze, known as the Skirball Fire, was one of several wildfires that have ravaged Southern California in recent days, spread by high winds and dry conditions.

Arson investigators went to the encampment a few hours after the blaze began in Bel-Air just before 5 a.m. last Wednesday. Although they found evidence that there had been people cooking and sleeping in the area, they found no one at the site. There are no suspects and it is unlikely one would be named, Mr. Sanders said.

“There was nobody when they got there and there is no video evidence or witnesses we know of,” Mr. Sanders said. “So, short of someone coming forward and confessing or another individual who may have been at the camp saying who did this, we will not have a suspect.”