A fire possibly sparked by a transient blackened about an acre on Tuesday on a brushy hillside near the junction of state Route 94 and SR-125.
The non-injury blaze began spreading off Grove and High streets in La Mesa shortly before 1:30 p.m., said Sonny Saghera, spokesman for Heartland Fire & Rescue.
It took firefighters about a half-hour to contain the flames, which never posed any structural threats.
A homeless woman seen fleeing from the area as the blaze began spreading was detained by sheriff's deputies for questioning, Saghera said.
The cause of the fire remained under investigation in the late afternoon.