A suspicious fire inside a large barn at Tolay Lake Regional Park was quickly brought under control Thursday by a large contingent of firefighters.

A half-dozen fire agencies responded to the 5 p.m. blaze at the park off Lakeville Highway east of Petaluma.

Park Ranger Scott Beck reported the blaze after he looked outside his home to see smoke pouring from the 6,000-square-foot barn, which is constructed of corrugated metal.

Beck said no one was inside the barn when the fire started and that the building was locked.

The barn housed equipment for county parks and the Lakeville Volunteer Fire Department.

Beck said a new John Deere tractor was inside the barn, along with tools, landscaping equipment and two motorized utility vehicles, including one that the fire department used for events at Infineon Raceway.

The barn was engulfed with flames by the time the first firefighters arrived, Sonoma County Fire Chief Mark Aston said. Firefighters had it under control within 20 minutes.

"There's a huge threat from this type of fire to the wildland, particularly at this time of year," Aston said.

He said there was no obvious ignition source for the blaze, which is under investigation.

Firefighters from Petaluma, Cotati, several volunteer fire departments and from Cal-Fire responded to the blaze.

Tolay Park consists of 1,769 acres of farm and grasslands, ridges, a seasonal freshwater lake, ponds and wetlands.