An investigation by BBC's Newsnight has cast doubts on the key piece of evidence which convicted the Lockerbie bomber, Abdelbaset Ali al-Megrahi. Tests aimed at reproducing the blast appear to undermine the case's central forensic link, based on a tiny fragment identified as part of a bomb timer. The tests suggest the fragment, which linked the attack to Megrahi, would not have survived the mid-air explosion. Two hundred and seventy people died in the 1988 attack on Pan Am flight 103. Megrahi was jailed for the attack in 2001, but he was controversially released from prison in Scotland by Scottish Justice Secretary Kenny MacAskill in August 2009 on compassionate grounds. Megrahi is said to be dying from terminal cancer and, according to reports from Libya, his condition continues to deteriorate. 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 UN European consultant on explosives, John Wyatt But his release also scuppered Megrahi's planned appeal and any hopes of challenging the evidence on which he had been jailed. Newsnight has been reviewing that evidence, and has exposed serious doubts about the forensics used to identify the fragment as being part of a trigger circuit board. The fragment was found three weeks after the attack. For months it remained unnoticed and unremarked, but eventually it was to shape the entire investigation. Malta link The fragment was embedded in a charred piece of clothing, which was marked with a label saying it was made in Malta. So the focus turned to Malta and the question of who had bought the clothes. A shopkeeper on the island identified Megrahi, but this came only years later after he saw him pictured in a magazine as a Lockerbie suspect. Newsnight has discovered that the fragment - crucial to the conviction - was never subjected to chemical analysis or swabbing to establish whether it had in fact been involved in any explosion. And the UN's European consultant on explosives, John Wyatt, has told Newsnight that there are further doubts over the whether the fragment could have come from the trigger of the Lockerbie bomb. Obliterated He has recreated the suitcase bomb which it is said destroyed Pan Am 103, using the type of radio in which the explosive and the timer circuit board were supposedly placed, and the same kind of clothes on which the fragment was found. In each test the timer and its circuit board were obliterated, prompting Mr Wyatt to question whether such a fragment could have survived the mid-air explosion. He told Newsnight: "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. "We carried out 20 tests, we didn't carry out 100 or 1,000, but in those 20 tests we found absolutely nothing at all - so I found it highly improbable that you would find anything like that, particularly at 10,000 feet when bits are dropping into long wet grass over hundreds of miles. Source: BBC.com