Wreaths were been laid at Dryfesdale Cemetery in Lockerbie to mark the 25th anniversary of the bombing of an aircraft above the town.
Memorial events are also to be held in the UK and the US to mark the attack in which 270 people were killed. The south of Scotland town was devastated when Pan Am flight 103 was blown from the skies in 1988.
At the time, the group was headed by Ahmed Jibril, a former Syrian Army captain, who was based in Damascus and funded by Iran.
Dr Swire suspects Iran was seeking revenge for the shooting-down of Iran Air Flight 655, with the loss of 290 lives, by the USS Vincennes in July 1988 – just five months before Lockerbie. Iran did not accept the US's claim that the incident was a mistake.
Dr Swire believes the families of the victims have been "pawns in a political scenario that had nothing to do with truth".
He added: "The Heathrow break-in would have been the perfect scenario for the Syrian-backed PFLP-GC to put one of its bombs on the aircraft. The grieving father also suspects that Mahmoud Ahmadinejad, who went on to become the anti-Western president of Iran but then reportedly an ambitious figure in Iran's Republican Guard, may have been involved in Lockerbie.
Another suspect is Abu Talb, a Palestinian later arrested and jailed in Sweden for terrorist bombings: he had circled the date of December 21 1988 in a calendar found at his flat. Other relatives, particularly those in the US seeking "closure" over the tragedy.
Critics say the evidence against Megrahi was, at best, circumstantial, and, at worst, flimsy. His lawyers have dismissed the evidence of Tony Gauci, a Maltese shopkeeper and the key prosecution witness.
He said he had sold clothes to Megrahi which had supposedly been in the suitcase that contained the bomb.
But it later emerged that Mr Gauci had seen a photograph of the suspect in a magazine linking him to the bombing. Other witnesses, it is alleged, were paid or offered money to give damaging evidence against the Libyan.
Dr Swire had controversially visited Gaddafi.