Fire investigators have not yet determined the source of a grass fire that blackened about a half-acre of brush around the perimeter of Kate O. Sessions Memorial Park on June 29. The cause of the flare up remains under investigation, according to Maurice Luque, a spokesman for the San Diego Fire-Rescue Department. Flames broke out about 2:15 p.m. Wednesday near the back of the park. Firefighters used a helicopter to make at least two water drops to help extinguish the blaze. Although firefighters took a precautionary stand near Edgeworth and Crosshill roads near neighboring homes, the structures were not threatened, according to fire officials. The grass fire was extinguished in about 45 minutes and no injuries were reported, said officials.