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

Officials inspect the wreckage of Pan Am flight 103 on 21 December 1988

I 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.

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.

Abdelbaset al-Megrahi was the only man convicted of the Lockerbie bombing

Jim 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.

"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 verdict 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.

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.

A mock-up of the explosives-loaded Toshiba c