Costa Mesa authorities were investigating three suspicious brush fires that broke out in Fairview Park and Talbert Regional Park on Wednesday afternoon.
The Costa Mesa Fire Department dispatched an engine at 3:23 p.m. to a small blaze near the bluffs in the northwest quadrant of Fairview Park.
Within 10 minutes, crews had doused those flames, but a few dozen square feet of brush where bikers use dirt mounds as a BMX course were scorched, authorities said.
As firefighters battled that blaze, they spotted plumes of smoke below the bluffs in nearby Talbert Park.
Two engines responded and found two more small fires, one with flames about 20-feet high.
Firefighters tore down a fence to get to the fires and had both under control in 10 to 15 minutes, officials said.
Battalion Chief Kevin Diamond, who led an arson investigation at the scene, called the fires "definitely suspicious," and police cordoned off the area in Talbert Park as a crime scene.
Police detained four people below the bluffs, but department spokesman Lt. Greg Scott said no arrests had been made as of 5:30 p.m. Wednesday.
Costa Mesa resident Catherina Oxford said she saw smoke from the first fire on the bluffs while she was walking through the park.
"I wasn't too sure whether I could believe my eyes," she said.
Bystanders quickly called for help, and a fire engine rumbled through the park to get to the fire, she said.
— Jeremiah Dobruck and Bradley Zint