Fire and police investigators are probing who brought two Molotov cocktails to Walden Park around 8:30 a.m. Wednesday.
One of the devices appeared to be exploded from having been hurled against a rock at the Oak Road park, said Vic Massenkoff, investigator for the Contra Costa County Fire Protection District.
The devices were found toward the back of the park, near the Iron Horse Trail. No injuries were reported.
Also investigating is the Walnut Creek Police Department.