In December 1988, Britain's worst terrorist attack left 270 people dead. After a massive police and FBI investigation, a Libyan intelligence officer was found guilty of mass murder, but he has now been granted a second appeal against his conviction. Twenty years on, will the full story of Pan Am flight 103 finally emerge? The Conspiracy Files follows the trail of evidence from the Middle East, through Europe and on to North America, investigating the tangled web of claims and counter-claims surrounding the Lockerbie bombing.

• Caroline Catz