One man is confirmed dead and authorities were searching for additional victims in a possible boating accident at Pyramid Lake near Castaic, according to the Los Angeles County Sheriff's Department.
A person on a jet ski flagged down a deputy about noon Sunday to inform him of a boat that was overturned or sinking at the north end of the lake, said Peter Gomez of the sheriff's headquarters bureau.
Deputies were able to pull an adult male from the lake but were unable to revive him.
Divers were called to the scene to search for any additional passengers who may have been on the boat, officials said.
Homicide investigators were also on site to determine if foul play was involved, or the death was accidental.
No additional information was immediately available.