According to the US Justice Department:
“In the winter of 1988, [Abu Agila] Masud was summoned by a Libyan intelligence official to meet at that official’s office in Tripoli, Libya, where he was directed to fly to Malta with a prepared suitcase. He did so, where he was met by Megrahi and Fhimah at the airport. After Masud spent approximately three or four days in the hotel, Megrahi and Fhimah instructed Masud to set the timer on the device in the suitcase for the following morning, so that the explosion would occur exactly eleven hours later.
"According to the affidavit, the suitcase used by Masud was a medium-sized Samsonite suitcase that he used for traveling. Megrahi and Fhimah were both at the airport on the morning of Dec 21, 1988, and Masud handed the suitcase to Fhimah after Fhimah gave him a signal to do so. Fhimah then placed the suitcase on the conveyor belt. Masud then left. He was given a boarding pass for a Libyan flight to Tripoli, which was to take off at 9:00 a.m.”
The bomb was a hidden plastic explosive concealed inside the audio cassette player within the cargo section, set to be exploded when the plane achieved 31,000 feet high altitude.
[RB: It appears that the author of the article cannot make up his/her mind whether the explosion was detonated by a pre-set timing device or by a barometric pressure device (or by both?).]