A bomb blew up Pan Am Flight 103 with 259 people aboard as the jumbo jet flew over Scotland a week ago, the Department of Transport announced Wednesday.

Investigators found conclusive evidence of a detonating high explosive in two parts of the metal framework of a piece of luggage, a statement said. "The explosive's residues recovered from the debris have been positively identified, and are consistent with the use of a high-performance plastic explosive."The statement did not speculate about who was responsible for placing the bomb aboard the plane or on a possible motive.

It was distributed in London as investigators convened a news conference in Lockerbie, the village that bore the brunt of the Boeing 747's flaming wreckage that fell out of the sky on Dec. 21.

The statement said evidence collected by the Transport Department's Air Accident Investigation Branch, including the cockpit voice and flight data recorders and the trail of wreckage on the ground, has led to the preliminary conclusion that the explosion took place soon after the aircraft had crossed the Scottish border while the jetliner was at 31,000 feet and that this led directly to its destruction.

"Much investigative work remains to be done to establish the nature of the explosive device, what it was contained in, its location in the aircraft, and the sequence of events immediately following its detonation," the statement said.

It said wreckage, baggage and part of the framework of a metal luggage pallet were being examined by Ministry of Defense scientists, while more items have been collected by Air Accident Investigation Branch investigators "will be subjected to lengthy chemical and metallurgical forensic examinations."

Earlier, British news reports said that heat damage to the plastic lining of a cargo bay and metal slivers found embedded in the bodies of passengers indicated that a bomb downed the jet.

In London, meanwhile, pine coffins containing the first bodies of Americans killed in the Dec. 21 crash arrived at Heathrow Airport and were loaded onto two separate Pan Am flights bound for New York.

In Scotland, authorities said they had formally identified two more victims: Michael Joseph Doyle, 30, of Voorhees, N.J., and Sean Concannon, 16, of Banbury, England.

Tuesday night, residents paid their silent respects to the five American victims of the jumbo jet disaster whose bodies were the first to leave this Scottish town since the New York-bound Boeing 747 fell onto it in pieces.

All 259 people aboard were killed, and 11 missing townspeople are presumed dead. Police spokesman Angus Kennedy said one more body was recovered Tuesday, bringing the total to 241.