Federal agents sifted through debris Thursday morning left by a bomb that ripped through a four-wheel-drive vehicle, critically injuring an 11-year-old boy Wednesday night.

Adam Cook, whose address has not been released, had been waiting in the vehicle for his father about 9:20 p.m. when he honked the horn, apparently detonating an explosive device that threw him from the car."Flames and sparks shot everywhere. I saw a body drop to the ground; he was blown out of the car, and smoke was rolling off of him," said Nikki Barlow, who had pulled her car 20 feet behind the Landcruiser seconds before the explosion.

The blast rained parts from the vehicle "all over my truck," she said.

Cook was flown by helicopter to University Hospital where he remained in the neuro critical care unit with head and eye injuries Thursday, according to hospital spokesman John Dwan.

The late-model Toyota Landcruiser was parked in front of 5740 S. 665 West, where Cook's father, whose name has not been released, had stopped to see someone at the house. The boy remained in the vehicle with the engine running, witnesses said.

The booming sound brought hundreds of residents to the area, some saying they could feel the explosion rattle their homes as far as two blocks away.

Salt Lake County bomb squad members and dogs scoured a block radius searching for other devices but found none. Police sealed off a two-block area and evacuated dozens of residents from the neighborhood, asking them to stay away for the night. Officers began escorting families back to their homes to get ready for work and school about 9 a.m. Thursday.

The investigation at the scene, under the direction of Federal Alcohol, Tobacco and Firearms agents, was suspended about 1 a.m. Thursday because of the difficulty of working under artificial lights, said Salt Lake County Fire Lt. Dennis Steadman. Officers returned about 6:30 a.m. to resume the investigation.

The entire block remained cordoned off for the rest of the morning while agents and bomb squad officers scoured yards near where the bomb exploded. Lawns were combed with metal detectors and the contents of garbage bags, left outside for collection that morning, were scattered in the street and inspected.

There was no information available Thursday morning about the kind of explosive or possible motives in the bombing.