British investigators believe the bomb that tore apart a Pan Am airliner over Scotland last month was probably smuggled aboard by a worker at London's Heathrow Airport, West German intelligence sources said Friday.

The sources said British investigators were working from the theory that the explosive was placed in a passage directly under the cockpit connecting it with the baggage hold.The plane exploded over the Scottish town of Lockerbie less than an hour after taking off from Heathrow on December 21, killing all 259 passengers and 11 people on the ground. The flight originated in Frankfurt.

The sources said the West German foreign intelligence service BND had been given the information by British investigators. It has been passed on to the government in Bonn.

They said British media reports that the bomb might have been planted in a passenger's luggage in a connecting flight from Frankfurt and then transferred to the jumbo jet at Heathrow were being increasingly discounted by British investigators.

West German officials say they have no evidence to suggest the bomb was planted in Frankfurt.

The London Times newspaper said on December 31 that investigators were certain that luggage containing a bomb had been transferred from the Frankfurt plane to the forward cargo hold of the Boeing 747 that left London.

The West German sources said that if the bomb had been planted in a suitcase, it would have taken at least 66 pounds to cause such an explosion.

Baggage weighing more than 44 pounds would not have been put in the forward freight compartment, they said, leading to suspicion that the bomb was strapped to the wall, ground or ceiling of the limited-access area.

The flight deck was ripped away from the rest of the plane and came down almost in one piece.