LONDON — A BBC investigation has cast doubt on key evidence in the case against the Libyan convicted of blowing up a US jet over the Scottish town of Lockerbie in 1988, the broadcaster said Wednesday.

A tiny fragment of the timer allegedly used to blow up Pan Am flight 103 -- crucial in linking Abdelbaset Ali Mohmet al-Megrahi to the bomb -- was not properly tested and was also unlikely to have survived the explosion, it said.

Megrahi was jailed in 2001 for the attack which left 270 people dead, but was controversially released from his Scottish prison in August 2009 because he was suffering from terminal cancer and only had months to live.

Investigators believe the plane bomb was contained in a Toshiba radio cassette player inside a brown suitcase with various items of clothing, and was triggered by a digital timer that was later linked to Libya.

But according to the BBC's Newsnight programme, the fragment of the timer -- found embedded in a charred piece of clothing three weeks after the bombing -- was never tested to confirm if it had actually been in a blast.

The BBC also quoted an explosives expert, John Wyatt, who recreated the suitcase bomb 20 times and found that each time, the timer and its circuit board were completely destroyed -- casting serious doubt on the fragment found.

"I do find it quite it extraordinary and I think highly improbable and most unlikely that you would find a fragment like that -- it is unbelievable," Wyatt, the UN's explosives consultant for Europe, told the programme.

It is not the first time doubts have been raised about the timer fragment -- in 1995, a British lawmaker suggested it could have been planted by the CIA.

A review by the Scottish Criminal Case Review Commission also cast doubt on evidence linking Megrahi to the clothes in the suitcase and concluded in 2007 that "a miscarriage of justice may have occurred".

Megrahi, who has always protested his innocence, subsequently launched a second appeal but dropped this in anticipation of his release.

The devolved Scottish government's decision to release him caused a diplomatic row with the United States, home to many of the victims, and the British government in London.