Iran has denied any involvement in the Lockerbie bombing in the face of new allegations it contracted Palestinian militants to carry out the 1988 attack which killed 270 people.
Documents obtained by Al-Jazeera television provided new backing to longstanding allegations that Iran, and not now-slain Libyan dictator Muammar Gaddafi, was behind the downing of the Pan Am airliner over the Scottish town.
"We reject any claims of Iranian involvement in this act of terror," said foreign ministry spokeswoman Marzieh Afkham.
"Iran's stance - not only on this case but on all terrorist-related issues - is quite clear: Iran flatly denies [links] to any act of terror."
Former Libyan intelligence officer Abdelbaset al-Megrahi - the only person convicted over the bombing - maintained his innocence right up until his death in May 2012.
Al-Jazeera said that new evidence gathered for Megrahi's planned appeal, which was aborted by his release from prison on compassionate grounds in 2012, supported his innocence and implicated a Syrian-based Palestinian militant group.
Campaigners led by Jim Swire, whose daughter was killed in the bombing, have long claimed that Tehran contracted the Popular Front for the Liberation of Palestine General Command to carry out the bombing in revenge for the shooting down of an Iranian airliner by the USS Vincennes, which killed 290 people in July 1988.
The Syria-based PFLP-GC is blacklisted as a terrorist group by both the EU and the United States.
In the documentary - Lockerbie: What Really Happened? - Al-Jazeera cites testimony from alleged former senior Iranian intelligence official Abolghasem Mesbahi, who defected to Germany in the late 90s.
Mesbahi claims Iran contracted the bombing to PFLP-GC leader Ahmed Jibril, and provides names of those he says were involved.
Former CIA agent Robert Baer, who was involved in the Lockerbie investigation, told Al-Jazeera that US intelligence agencies had long been convinced of Iran's involvement.
He said Gaddafi's Libya was blamed because the US Government did not want to alienate Syria in the run-up to the 1991 Gulf war.
Gaddafi's regime admitted responsibility for the Lockerbie bombing in 2003 and eventually paid US$2.7 billion in compensation to victims' families.
But Gaddafi's now jailed son Seif al-Islam has long insisted that the admission was a tactical ploy to end the regime's pariah status and mend fences with the West.