Police received several calls around 3:50 a.m. about a fire on a state Highway 29 overpass south of Trancas Street, Capt. Jeff Troendly said.
Firefighters found a man on fire and extinguished the blaze, but he was pronounced dead at the scene, Troendly said.
The death is initially being investigated as a homicide but Troendly said it might be a suicide.
The man's identity is unknown, and police have not received any missing-person reports, Troendly said.
Police also visited homeless camps but have not learned of any transients who have gone missing, he said.
Investigators were still at the scene mid-morning.