The shooting was reported around noon near East Fork Road, just north of the San Gabriel Reservoir, according to the Los Angeles County Sheriff's Department.
No one was reported struck by the gunfire, which prompted a massive law enforcement response from deputies on the ground and three sheriff's helicopters above the scene.
A search for the gunman ensued and Glendora Mountain Road was closed in the area as the investigation continued.
Shortly before 1 p.m., two deputies who had been lowered to the ground from one of the helicopters detained a suspect. Details about the person in custody were not immediately disclosed.