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. Newsnight 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. 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.