Detectives Thursday said they would like the bicyclist to contact the department to give a more detailed statement.
The fires were reported in the grasslands of the wooded preserve west of Interstate 280 at 4:08 p.m. and came within about 200 feet of a horse ranch on the adjacent Stanford University campus. No structures were threatened, and no one was injured.
Madeleine Todd, the owner of three horses at Portola Pastures, reported the fire after she noticed smoke and flames from four separate blazes, she said.
Todd said she was checking on her horses at the time. The fire was near the parking lot of Palo Alto University on Arastradero Road, she said.
Todd said she saw three, equally spaced fires near the road. A fourth, a larger blaze, was farther in the grass up a hill near a trail.
When she called in the fire, people were still biking and jogging along the road. The fires had probably been burning for 10 to 15 minutes, she said.
"It was so big by the time the fire trucks got here, there was lots of smoke and about 15-foot flames. It took down a couple of trees," she said.
She and Portola Pastures manager Jose Ruelas were on alert in case they needed to move the 15 or 20 horses out of the upper pasture, she said. Ruelas said Portola Pastures shelters about 155 horses.
Firefighters from Palo Alto, Menlo Park, Woodside, Coastside in Half Moon Bay, Santa Clara County and Cal Fire, plus aircraft that dumped fire retardant and a helicopter that dropped water, swooped down on the blaze, according to Cal Fire.
Police blocked traffic along Arastradero Road from Page Mill to Alpine roads and closed the preserve. Only horse owners were allowed to check on their animals.
Crews had the fire contained by 6 p.m. and remained into the evening to ensure the hot spots did not flare.
The open-space preserve is open, with all hiking trails available, according to the police statement.
Police are asking anyone with information about the fires to contact the department at 650-329-2413. Anonymous tips can be emailed to firstname.lastname@example.org or sent via text message or voicemail to 650-383-8984.
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