The bomb that blew up Pan Am Flight 103 on Dec. 21 was planted in a radio-cassette player and was probably loaded on the plane in Frankfurt, the detective heading the investigation said Thursday.

Chief Superintendent John Orr refused to describe the size of the recorder and he said it was not certain the bomb was put aboard in Frankfurt, where the flight originated.But answering reporters' questions, he said "the balance of probabilities" pointed to Frankfurt rather than London, where the New York-bound flight changed jets and took on new passengers.

All 259 people on board the plane and 11 people on the ground were killed when the Boeing 747, flying at 31,000 feet, exploded over Lockerbie.

On Feb. 8, the Jerusalem Post reported the bomb that destroyed the jet was hidden in a radio-cassette recorder and was traced to Frankfurt.

That report quoted unidentified investigators as saying the device was similar, but not identical, to one found earlier in the possession of members of Ahmed Jibril's extremist Popular Front for the Liberation of Palestine-General Command.

The group has denied involvement in the blast.

"While there is insufficient evidence at this stage to establish the identity of the person or group responsible for this dreadful crime, the progress made and the evidence obtained has been substantial," Orr told a news conference. Asked whether the discovery would direct the investigation to a particular country, Orr said: "It may."