I thought the Lockerbie bombing was a closed case until I started asking questions

Image source, Getty ImagesImage caption, Officials inspect the wreckage of Pan Am flight 103 on 21 December 1988ByMyles BonnarBBC DisclosurePublished17 minutes agoI was only a year old when the UK's deadliest terror attack shocked the world.In December 1988, Pan Am flight 103 exploded over the Scottish town of Lockerbie, killing all 259 people on board as well as 11 on the ground.I was a teenager by the time Libyan intelligence officer Abdelbaset al-Megrahi was convicted of the bombing in 2001 and, when he later died, I thought that was largely the end of the story.

It wasn't.The case still rumbles on and, later this year, a man suspected of making the bomb which blew up the plane, Abu Agila Masud, goes on trial in Washington DC.As part of a Radio 4 Intrigue podcast series, I had an opportunity to delve into this complex case with fresh eyes.

The deeper I dug, the more complicated Lockerbie became and there have been times that I don't know what to believe.Image source, Getty ImagesImage caption, Abdelbaset al-Megrahi was the only man convicted of the Lockerbie bombingJim Swire, on the other hand, is sure what he believes - and it is not the official story.His daughter Flora died in the bombing, but he is convinced that whoever killed her got away with mass murder.

Swire believes Megrahi is innocent, even befriending the convicted bomber up until his death in 2012."The burning issue was to get to the truth," he says."Always has been, always will be and I will not shut up about what I know to be true."

Swire's doggedness has made him a prominent voice but most of the Lockerbie victims' families don't agree that Megrahi's conviction was a miscarriage of justice - and the bomber’s own appeals against the verdi either failed or were abandoned.For Swire, the simplest explanation is that Iran was behind the attack - not Libya.The Lockerbie bombing came five months after 290 people were killed when an Iranian commercial airliner was shot down by a US battleship.

Iran swore revenge and months later Pan Am flight 103 exploded on its way to New York.Image source, Getty ImagesImage caption, Jim Swire's daughter Flora died in the Lockerbie bombingAnd there's more.Iran - much like they do now - used proxy militant groups to pursue their ends.

Before the Lockerbie bombing, a militant cell based in Frankfurt, part of the Popular Front for the Liberation of Palestine - General Command (PFLP-GC), was foiled by German police.Police arrested suspects and seized explosive devices, concealed in radio cassette players, that were designed to trigger at altitude.The bomb that brought down Pan Am 103 was in a radio cassette player hidden inside a samsonite briefcase.

Because of this, Swire believes the device that brought down Pan Am 103 was loaded at Frankfurt, and not Malta as the court had concluded.Image source, Getty ImagesImage caption, A mock-up of the explosives-loaded Toshiba cassette recorder which blew up Pan Am Flight 103And, for Swire, this is only the tip of the iceberg.He also questions much of the evidence presented at the 2001 trial, discredits witnesses, and is extremely suspicious of the involvement of intelligence agencies - like MI5, MI6 and the CIA - in the aftermath of the bombing.

Combine these suspicions with his all-consuming grief and you can see why Swire is such a force to be reckoned with on his quest for a truth he believes has been denied to him."Discovering the truth became my way of handling the consequences of the grief reaction," Swire says."And it was very tough for other people to understand that but that's just the way I'm made, I suppose."

Intrigue: LockerbieOn 21 December 1988, a plane explodes over Lockerbie, Scotland, killing 270 people. Almost four decades on, there's fresh evidence. Who was really behind the bombing?

Listen on SoundsDick Marquise, who led the FBI's Lockerbie task force, says that in the early days of the case he too believed the PFLP-GC were the "number one suspect", but years of investigation found nothing to prove it."There was not one shred of evidence that could be taken to court and get a conviction," he says. "You couldn't do it.

There was nothing."However, there are others, besides Swire, who still question the "official narrative" - including former CIA agent John Holt who also believes Iran was behind the bombing.Holt worked with Libyan intelligence assets in the 1980s and was the handler of double agent Abdul Majid Giaka, whose information became central to the investigation into the Lockerbie bombing.

Holt now believes Giaka was a "BS artist" who was out for money and was feeding the CIA fake stories to get paid."I didn't think much of him at all," he says."He gave us some little tidbits about what was going on, but he never knew anything about the Pan Am bombing.

And he knew a lot about Megrahi," Holt says.Holt believes the fact the CIA bankrolled Giaka devalues him as an informant."It makes it easier for somebody to say whatever the person who's giving them money wants