Mercer Island police and fire officials are investigating a fire at the South Mercer Playfields on Sunday morning that was reported by an early morning walker at 5:33 a.m.

Engine 92 from the South Mercer Fire Station responded to the fire, said Fire Investigator Rob Villalobos, who was examining the scene on Monday morning.

“They extinguished it pretty quick, but it was fully engulfed when they arrived,” he said.

Contractor Tim Collins said this is the crew’s fourth week at the site, and he’s never had problems with theft or fire at other job sites. Advanced Construction was hired to remodel the bathrooms, build a concession stand and add a scoreboard to the park.

The trailer was locked and contained a number of tools and equipment, all of which were destroyed in the inferno, he said. The total loss is about $15,000.

According to the police report, a lock on the chain link fence surrounding the trailer and other heavy equipment was damaged — but still locked — and a section of the fence had been pulled from a cement anchor.

On Monday, Villalobos was busy sifting through the charred contents of the trailer in search of any evidence of how the fire started.

“We don’t have an ignition source yet,” he said. “It’s still under investigation.”