Investigators on Monday determined flames were ignited near the public trail off Communication Hill Boulevard, which sparked a two-alarm grass fire late Sunday.
The blaze consumed between 5 and 7 acres of vegetation, firefighters said.
San Jose Fire Department Captain Michael Van Elgort said the fire area in the hills was tough to see and hard to catch.
"This terrain made it more challenging," he said.
Investigators said flames destroyed any evidence to help lead them to what sparked the blaze.
Resident Tom Hess, who lives near the area, said he heard explosions.
"The engulfment turned into explosions," Hess said.
Elgort said there were also reports of fireworks in the area.
"I would hate to say this was arson," Elgort said.
Fire crews responded to the blaze at around 9:30 p.m. in the area of Curtner Avenue and Communications Hill Boulevard.
Firefighters declared the blaze "knocked down" at 10:39 p.m.
No injuries were reported in the fire, Elgort said.
Bay City News contributed to this report.