“All I was after was the truth,” says Dr Jim Swire.

The retired GP’s 35-year search for answers has seen him board a US-bound flight from Heathrow carrying a replica bomb, hold a secret meeting with Libyan dictator Muammar Gaddafi, and collapse in shock after a criminal trial at a former military base in the Netherlands.

His 23-year-old daughter was among the 270 people killed in the Lockerbie bombing on 21 December 1988 – the deadliest ever UK terrorist attack.

“I think I know who was responsible for killing her and I think I can prove it,” the old Etonian, now 87, says in a new four-part Sky documentary.

He keeps the evidence he has collected in cardboard folders in a metal filing cabinet in an office in the Cotswolds home he shares with his wife Jane.

‘No one had really heard of Lockerbie’

Flora “was everything a parent could wish for”, says Mrs Swire.

She was about to turn 24 and studying medicine when she set off to the US to meet her boyfriend for Christmas.

“Everything was booked up, except there were plenty of seats available on a certain flight known as Pan Am 103,” says Dr Swire, sitting in a leather armchair in his cottage, overlooking the rugged coastline on the Isle of Skye.

Less than 40 minutes after taking off from Heathrow on the transatlantic leg to New York’s JFK, the Boeing 747 was 31,000ft over the Scottish town of Lockerbie when the aircraft was almost instantly destroyed by a massive blast.

Residents remember “a huge explosion” before the sky lit up with “bright red flames” and a “great big mushroom ball of fire”.

“Before 1988, no one had really heard of Lockerbie,” says Colin Dorrance, who was a 19-year-old recruit just three months into his police career at the time.

“Life here was just undramatic.”

That all changed at 7.03pm that evening. All 259 passengers and crew members on board the plane were killed along with 11 people in the town as windows were blown in and wreckage destroyed their homes.

Locals are still haunted by images of the bodies that fell from the sky, some still strapped in their seats as they landed in gardens and fields.

The smell of aviation fuel hung thick in the air as they surveyed the carnage strewn with luggage and the Christmas presents victims were carrying for loved ones.

Peter Giesecke can’t shake the image of the woman still wearing one high-heeled shoe, while Margaret and Hugh Connell became “attached” to the man they found in a field near their home, watching over him for 24 hours until his body was recovered.

“We developed quite a love for ‘our boy’, not knowing who he was,” says Mr Connell.

As news of the disaster broke, relatives were desperate to know whether their loved ones were on board.

Unable to get through to Heathrow, Dr Swire rang the Pan Am desk in New York and could hear “chaos in the background and women screaming” as families of the victims, many of whom were American, received the terrible news.

Dr Swire, tall and slim with a full head of white hair, is measured as he recalls the kindness of the pathologist who allowed him to see his daughter’s body in the local ice rink, where the post-mortems were being carried out.

“She was barely recognisable,” he says, the grief which still bubbles just under the surface after all these years coming to the fore as he tells how he was allowed to take a lock of Flora’s hair.

“Human kindness can be very important when these things happen,” he adds, with tears in his eyes.

‘Nothing quite adds up’

It took investigators a week to discover the disaster was caused by a bomb in a terrorist attack against the US – the biggest in the country’s history until 9/11.

“My first reaction was of fury, which led me to want to find the truth,” says Dr Swire. And that did a lot to help with the grief because I was busy doing things. It was rather how, I think, Flora would’ve reacted.”

The prime suspect was Iran, but they have always denied any involvement in the attack.

Iran had vowed to take revenge for the accidental downing of an Iran Air passenger flight by the US Navy in the Gulf in July 1988, which killed 290 people.

But the sprawling international investigation was just beginning.

“Nothing is what it seems in the Lockerbie story, nothing quite adds up,” says local reporter David Johnston, one of the first journalists on the scene.

It soon emerged a call was made to the US embassy in the Finnish capital that a Pan Am plane from Frankfurt to the US would be bombed in what was known as the “Helsinki warning”, with American diplomats in Europe told of a threat.

Passengers and luggage were transferred at Heathrow to Pan Am 103 from a feeder flight originating in Frankfurt and Dr Swire believes the plane was only two-thirds full because people were “warned off”. “We weren’t warned. Nobody told us,” he says.

“I felt I had a right to know the truth about how my daughter had come to be killed and why she wasn’t protected against being killed. And those were the bases on which we very soon found we were being richly and profusely deceived by the authorities.”

The ‘biggest crime scene in history’

Wreckage from the plane was spread over 845 square miles in what Richard Marquise, who headed up the FBI Lockerbie taskforce, describes as “the biggest crime scene in history”.

Investigators concluded the bomb was in a cassette player that was wrapped in clothes and put inside a brown hard-sided Samsonite suitcase.

A fragment of Toshiba circuit board pointed to possible links to tape recorder bombs made by Iran-backed PLFP-GC, a Palestinian terror group active in the 1970s and 1980s, who were suspected of carrying out the attack for the Iranians.

Dr Swire took his own replica bomb – the explosive material substituted for marzipan – on a plane from Heathrow to the US to highlight the security flaws.

“It was an obsession,” he admits. “All I was after was the truth of why our beautiful daughter had been murdered and I was bloody determined to find out who did it.”

The kindness of the women in Lockerbie

Meanwhile, in Lockerbie volunteers were cleaning the mud, blood and aviation fuel from the victims’ belongings left scattered amid the wreckage and bodies.

Clothes were washed, pressed and folded, jewellery was polished, and the pages of a tattered bible were individually ironed.

Miami-based Victoria Cummock, whose husband John died on board, was surprised to receive his clean laundry.

“I got back his personal effects due to the kindness of the women in Lockerbie,” she says.

The Malta connection and the Libyans

Charred clothes which were packed with the bomb were traced to a shop in Malta, and two Libyan suspects came into the FBI’s sights.

Colonel Gaddafi’s Libya had a motive for the attack after an American bombing in capital Tripoli and a tiny fragment of circuit board, called PT35, found embedded in a shirt collar 20 miles from Lockerbie, was traced to Swiss electronics expert Edwin Bollier, who said he sold a batch of timers to the rogue state.

After CIA asset Majid Giaka, a Libyan double agent codenamed “Puzzle Piece”, said he saw the suspects with a brown suitcase at Malta airport the day before the bombing, two men were charged.

But there was little hope of Colonel Gaddafi handing over Abdelbaset al Megrahi, a Libyan intelligence officer, and Lamin Khalifah Fhimah, a security official for Libyan Arab Airlines, to face trial.

Telling only his wife for fear he would be intercepted by the security services, Dr Swire travelled to Libya to meet the dictator face to face in an attempt to persuade him.

“I was pretty crazy at that time,” he says. “I was so determined that I wasn’t scared, nervous yes, but not scared.”

Dr Swire says he heard the “click, click, click” of Gaddafi’s female soldiers readying their AK47s as he opened his briefcase to reveal pictures of his daughter, then again at the end of the meeting when he pinned a badge reading “Lockerbie the truth must be known” to the Libyan leader’s lapel.

The meeting had no obvious impact, and it was not until 11 years after the bombing that Gaddafi finally agreed to extradite the suspects in the face of tough economic sanctions imposed in response to the atrocity.

‘The shock was so great I collapsed’

The trial was held at former US Airforce base Camp Zeist, in the Netherlands, under Scottish law, and Dr Swire rented an apartment with Rev John Mosey, whose 19-year-old daughter Helga died on board Pan Am 103, to follow the evidence closely over 84 days.

Supergrass Giaka crumbled in the witness box as he was shown to be a liar and a fantasist, while Bollier couldn’t confirm he supplied the bomb timer to Libya.

“I couldn’t continue to believe that there was a cogent body of evidence that justifies the finding of either of those two men guilty,” says Dr Swire.

The Scottish judges cleared Fhimah but found al Megrahi guilty of 270 counts of murder for which he was later handed a life sentence.

“The shock of the verdict initially was so great I collapsed,” says Dr Swire.

Families of the American victims were pleased with the guilty verdict and FBI agents felt vindicated by the finding Libya was behind the bombing.

But Dr Swire “couldn’t believe three senior Scottish judges could convict someone on that evidence”, which he believes to be “false” in order to frame Libya to protect the West’s fragile relationship with Iran.

“I wasn’t prepared to have anything associated with Flora’s death as untrue and debasing as the story that was raised by the authorities against those two men,” he says.

“I was very shaken up psychologically by the fact I knew al Megrahi was innocent, and the authorities protected her killers.”

A spokesperson for the Crown Office and Procurator Fiscal Service said: “The bombing of Pan Am 103 is, to this day, the deadliest terrorist attack on UK soil and the largest homicide case Scotland’s prosecutors have ever encountered, both in terms of scale and of complexity. Our thoughts are always with those who lost loved ones.

“The evidence gathered by Scottish, US and international law enforcement agencies has now been tested in court at both trial and at appeal three times, and the conviction of Megrahi stands.

“For over 30 years Scottish police and prosecutors have continued the search for evidence. This work has not yet concluded and there remain Libyan co-conspirators under active investigation.

“In December last year the US Attorney General announced charges against Abu Agila Mohammad Masud for his role with Megrahi and others in the attack, and we continue to work with US colleagues to assist and support their preparation for trial.”

‘The truth is very simple’

In 2009, al Megrahi was released from a Scottish jail on compassionate grounds after he was diagnosed with terminal prostate cancer, having spent just nine years behind bars.

But some believe he was freed in exchange for an oil deal with Libya.

He received a hero’s welcome when he landed back home with Scottish flags waved as he got off the plane.

Families of the American victims were disgusted but Dr Swire was happy and even visited him before he died in 2012.

From his Zurich office, Mr Bollier now claims the PT35 fragment is a fake and says he believes police tampered with the evidence.

He also says he was shown a brochure with two briefcases full of cash and offered $4m (£3.2m) by Mr Marquise, but the ex-FBI agent insists he didn’t offer him “one cent”.

For Dr Swire “the truth is very simple but the consequences of trying to conceal the truth are very complicated”.

“I think she (Flora) was killed by a bomb which was ordained by the Iranian authorities,” he says.

“They had had an Airbus destroyed by an American missile and 290 people killed. Therefore, they were lusting for revenge.”

Former CIA operations officer John Holt, the one-time handler of Giaka, agrees. “I have no doubt it was Iran,” he says, adding that the PLFP-GC carried out the attack on their behalf.

However, most people still believe the official narrative and Libya has officially accepted responsibility, agreeing to a $2.7bn (£1.95bn) compensation deal with the victims’ families, albeit with expectations sanctions would be lifted.

Dr Swire’s search for answers continues as the alleged bombmaker Abu Agila Mohammad Mas’ud Kheir al Marimi is in US custody awaiting trial accused of being the third man involved in the terrorist attack.

Back in Lockerbie, the Connells did find out who their “boy” was – New Yorker Frank Ciulla.

The couple have formed a lasting friendship with his widow Mary Lou Ciulla and daughter Michelle Ciulla Lipkin, who are greeted with warm smiles and hugs as they step into their home from the Scottish drizzle.

“I felt that he was alone somewhere and yet when I came here, he wasn’t alone,” says Mrs Ciulla, her friend Mrs Connell’s arm around her shoulder. “Mine was actually… a nice story.”