Two 17-year-old Loudoun County youths were arrested early yesterday after police discovered a homemade pipe bomb and highly explosive materials in the car they were driving, county officials said.

The pipe bomb was found shortly after a Loudoun sheriff's deputy had pulled the car over on a routine traffic stop at 1:33 a.m. yesterday on Palisades Parkway near Route 7 in the Cascades subdivision.

When the officer looked in the vehicle, he noticed "a homemade explosive device" and other explosive materials, said Barbara Barton, spokeswoman for the Loudoun Department of Fire and Rescue Services.

Loudoun officials called in the Fairfax County police bomb squad to secure the bomb and liquid and powder explosives found in the car. Authorities at the scene reported that if the bomb and explosives had been detonated, the blast "could have injured or killed anyone in a one-quarter-mile radius," Barton said.

Fairfax bomb technicians destroyed some of the explosive materials at the scene, Barton said. No injuries were reported.

Additional explosive materials were found in a car parked in the 200 block of West Meadowland Lane in the Sugarland Run area of Loudoun, in front of the home of one of the youths, authorities said. Police also searched the house, but no explosives were seized, Barton said.

One of the youths was held yesterday afternoon at a state juvenile detention center pending a hearing today on two charges each of felony manufacturing of an explosive device and possession of an explosive device, said Lt. Dave Kerr of the Loudoun Sheriff's Department. The charges were filed jointly by the sheriff's department and the fire marshal, Barton said.

The second youth was charged with an alcohol violation and released to his parents pending a hearing, Kerr said. Authorities interviewed yesterday said they were not sure whether the boy was drinking or whether alcohol was found in the car.

The youths' names were not released because of their ages.

A joint investigation is being conducted by Loudoun County detectives and the Fire Marshal's Office, Barton said.