A fire of unknown origin early Monday morning destroyed a construction trailer inside the Haunted Trail site in Balboa Park.
The blaze at 3 a.m. spread to some eucalyptus trees and tents at the Halloween site, but no injuries were reported.
San Diego Fire-Rescue crews found the trailer fully engulfed in flames when they arrived at the scene at Balboa Drive and Juniper Road.
The Metro Arson Strike Team is investigating the cause of the fire.
The annual holiday attraction opened Sep. 24, but firefighters said it would likely be closed today.
City News Service and OnScene.TV contributed to this article.