Two teen-aged boys tripped a Vietnam-style booby trap hidden along a wooded trail near Dale City this weekend, setting off a blast that rocked houses hundreds of yards away but left the teen-agers with only minor powder burns.

"It was a hell of an explosion," said Cpl. Richard Abel of the Prince William County police department yesterday. ". . . The person who set it really knew what he was doing."

Police have begun a hunt for a man in military fatigue dress recently seen pointing a gun at children in the area. They believe the bomb was the work of an expert.

"Dirt and smoke and flames shot up in my face," said one of the boys, Vince Kotrady, a 10th grader at Gar-Field Senior High School. His companion and classmate, Brett Baugh, fell to the ground and shielded his ears. Thinking that someone was shooting at them, the two sprinted in a panic to nearby Calexico Lane.

But when police searched the trail, which is frequently by children, they found a foot-wide crater, a tripwire made of clear fishing line and trees scarred by shrapnel from what was believed to be a battery-detonated pipe bomb. One infant hickory tree was sheared off two feet above the ground. p

Police said the boys could have been killed had the shrapnel been directed toward them. On Saturday, officers roped off the woods and searched without results for more bombs. Demolition specialists were scheduled to return to search for more fragments.

The incident created nervousness in Dale City, a sprawling residential tract. The woods on its fringes, with a stream and underground "fort," are a favorite haunt of local children. "All the kids in the neighborhood are afraid sombody's going to be killed," said Harold Beer, who lives with his six children at the edge of the woods.

Police could offer no motive for the explosion, which occurred late Saturday morning along a trail marked with logs leading toward the fort. But Cpl. Albe noted that on three separate occasions in recent weeks residents have reported seeing an armed man dressed in military camouflage fatigues in the woods.

At least once he reportedly approached a baseball lot at Dale City's Cloverdale Park and pointed a rifle or shotgun at children there. Police searches at the time found nothing, Abel said.

Police also noted that Dale City residents had reported hearing two other explosions on the morning of the incident. According to Richard Sapp, a computer programmer who lives near the wooded area, deer and other wildlife are common there and gunshots regularly ring out from the woods.

Cpl. Abel said the force's demolition expert, Officer Michael Null, compared the bomb to devices he had seen as a soldier in Vietnam. The tripwire was strung across the trail at a height of about 18 inches. One end of the line fitted into a triggering device made from a clothespin that had been nailed to a tree.

Tripping the wire completed an electical circuit. Wires led down to the bomb and battery, which police believe were covered over with dirt alongside the trail. That covering may have helped deflect the shrapnel away from the two boys, Abel said.