LOS ANGELES, Oct. 31 — A 10-year-old boy admitted that he accidentally started one of the largest of last week’s Southern California wildfires while playing with matches, enforcement officials say.

The blaze, the Buckweed fire, started in the early afternoon of Oct. 21, in Agua Dulce, a rural community in the northern part of Los Angeles County. Fanned by high winds and hot, dry weather, it spread quickly, driving 15,000 people from their homes, destroying 21 houses and 22 other buildings, injuring three people and blackening more than 38,000 acres.

Investigators immediately began tracing the cause of the fire, and on Oct. 22 arson investigators from the Los Angeles County Sheriff’s Department talked with the suspect, whose name has not been released, a department spokesman, Steve Whitmore, said Wednesday. “It became known to them that they needed to speak with this young boy,” Mr. Whitmore said. “He acknowledged that he was playing with matches and accidentally, his words, set the fire.”

Mr. Whitmore would not discuss the case further before evidence was presented to the county district attorney’s office.

The youth was left in the custody of his parents, awaiting word on whether he would be prosecuted. If so, the case would be heard in the county’s juvenile justice system, said Sandi Gibbons, the public information officer for the district attorney’s office.

“We have had cases involving minors this young before” Ms. Gibbons said, recalling the case of a 10-year-old boy who was prosecuted for killing a bicycle-shop owner.

It was not clear Wednesday whether the boy’s parents could be held financially responsible for damage caused by the blaze, which has been fully contained.

Several adult arson suspects have been arrested in the seven counties affected by last week’s fires. “A 10-year-old boy is in a whole other psychic realm,” said Dr. Jeff Victoroff, associate professor of clinical neurology and psychiatry, at the University of Southern California. “At least one study suggests that if you take a population of boys between kindergarten and fourth grade, 60 percent of them have committed unsupervised fireplay, which is to say that fireplay is a common and absolutely normal part of human development.”