Flora Swire is everywhere in her parents’ home. There are sketches and photos of her pinned to a board in the kitchen, on the mantelpiece, on the cover of a book; her portrait fills the wall across from their bed. There remains too a lock of her hair – a heartbreaking keepsake taken when the Swires saw her last, almost 35 years ago, after a bomb exploded beneath her feet in the Lockerbie disaster.

It was on 21 December 1988, the eve of her 24th birthday, that Flora, a promising neurology student who had just been accepted to do a PhD at Cambridge, took her seat on a plane bound for New York. She had hoped to spend Christmas with her boyfriend, but would never make it.

Thirty-eight minutes after taking off at Heathrow, Pan Am Flight 103 exploded in the sky over the town of Lockerbie in Dumfries and Galloway, with such force on a windy night that the debris landed across an 845-square-mile radius from southwest Scotland to the east coast of England. The fairylights on Christmas trees all over Lockerbie blew their fuses, along with the rest of the grid; smoking orange flames illuminated the town, which quickly filled with the stench of jet fuel.

All 259 passengers were killed, along with 11 residents. Bodies were found on hedgerows and limbs were discovered on the golf course of this quiet farming town, which would instantly become a byword for the worst terrorist attack in British history.

The investigation has remained open ever since, with one man, Abdelbaset al-Megrahi, a Libyan national, the only person ever to be convicted of the atrocity. He was convicted in 2001 and given a life sentence, and died in 2012. But in February this year, the case returned to the courts for the first time in more than two decades.

Another Libyan national, Abu Agila Mohammad Mas’ud Kheir Al-Marimi (known as Mas’ud) has been accused of making the Lockerbie bomb, and is now awaiting trial (he has pleaded not guilty). The development should offer some shred of hope for the families whose lives irreparably changed that night. Yet Dr Jim Swire, Flora’s father, ‘has no interest’ in the prospect of Mas’ud’s conviction.

‘I know he didn’t make the bomb,’ Jim tells me. ‘I know who made the bomb.’

As such, the official criminal verdict on events to date – upcoming trial included – is, in his view at least, nothing more than ‘twaddle’.

Jim, now 87, had been writing Christmas cards on that December night in 1988 when his wife Jane told him that a plane had just come down over Scotland. He tried calling Heathrow, where Flora had been dropped off by her younger sister, Cathy, a few hours earlier – he spent five hours on hold to Pan Am as news coverage blared, showing body parts hanging from a roof, the 30ft hole a chunk of the 747 had left in a Lockerbie street, and relatives howling in anguish at JFK Airport. When he finally got through, staff confirmed the worst possible news: Flora had been on the flight.

‘The body goes into shock,’ Jane remembers. ‘There’s almost disbelief. But underneath the disbelief is that awful feeling that this has happened.’ That feeling would only persist as the days went on, as the bodies intact enough to be collected were laid out in Lockerbie’s ice rink, where Jim asked to see Flora one last time. Flora’s face had been badly damaged but a look at her big toe, and its instantly recognisable pigmented spot, left no room for error.

Jim, an old Etonian who went to Cambridge, is still spry in his late 80s – part-raconteur, part activist, wearing a sharp grey suit and trainers. Today, Jim, who became a GP but ultimately left the profession after his daughter’s death, and Jane, 84, take turns bustling between the kitchen and back garden of their home in the Cotswolds town of Chipping Camden with offers of cheese sandwiches and cups of tea. It is a cosy idyll that conceals the sea of names and dates and evidence-tag numbers still etched on their minds.

Some 35 years on, the Swires’ agony remains barely beneath the surface, the memories of their eldest child both a precious gift and cruel reminder of what they have lost. ‘To lose a close family member gives you a life sentence immediately,’ Jim says. ‘Your whole life is altered. And you have to start asking yourself how, how can you go on living, or how can Jane go on living, with a loss so terrible as this?’

Their experiences are documented in Lockerbie, a new four-part documentary that airs on Sky next week. It is a panoptic watch, following the lives of the residents in the town that was, until that day, just a fish ’n’ chip pitstop, 75 miles from Glasgow, before it was completely upturned. The documentary follows the families of UK and US victims, and officials from across the town’s police force, the FBI and the CIA, too. But it also lays bare how devastation led to remarkable acts of humanity, as residents mounted a volunteer effort to wash the clothes and teddies scattered thousands of miles from where they should have ended up, and sent them back to passengers’ loved ones; some of which resulted in relationships with grief-stricken families an ocean away that remain strong. Their lives are, now, forever intertwined.

But underlying the heartfelt stories is a darker thread – for decades on, opinions about who was to blame for the disaster are more divided than ever.

Jim remains dismayed by what he sees as a 35-year-long miscarriage of justice. In the immediate aftermath of the disaster, he became the spokesperson for the UK Families Flight 103 group and in the intervening decades, he has met numerous experts and officials, and had independent reviews of evidence undertaken. All of which has convinced him that justice has not been served – and that the wrong man was imprisoned, just as another ‘wrong man’ is now about to be tried.

His theory – that Libya wasn’t responsible for the bombing – runs counter to al-Megrahi’s conviction and Mas’ud’s arrest, and has been dismissed by many. But there are others in his corner, too. ‘Enough honest, reliable and knowledgeable people have discovered the awful truth behind this to know that the truth will now be able to look after itself,’ Jim says. ‘If I die tomorrow, I know the truth will eventually come out.’

Among those people is former CIA investigator John Holt, the long-time handler for the principal US government witness at al-Megrahi’s trial, Libyan agent Abdul Majid Giaka. Holt said at the time that Giaka never provided ‘any evidence pointing to Libya or any indication of knowing anything about that nation’s involvement in the two years after the bombing’ – despite later testifying. But when accused of lying under cross-examination, Giaka replied: ‘I had no interest in telling anybody any lies.’

Others who have been vocal about what they view as Libya’s wrongful implication include solicitor Clare Connelly, director of the Lockerbie Trial Briefing Unit, an independent project established by the School of Law of the University of Glasgow, and other UK relatives, including John Moseley, whose 19-year-old daughter Helga was killed on Flight 103.

Al-Megrahi’s trial took place 22 years ago at Camp Zeist, a Scottish law court set up in the Netherlands (deemed a neutral territory), where judges heard that he had placed a bomb in a Samsonite suitcase. Lamen Khalifa Fhimah, his co-accused, was acquitted.

There was no smoking gun for the prosecution, but al-Megrahi was found guilty based on a series of links they felt couldn’t otherwise be explained: including that he had an office in Switzerland down the hall from a clockmaker whose device was used to make the bomb; and that clothing fragments found alongside remains of the bomb were traced back to a Maltese shop that its owner, Tony Gauci, said al-Megrahi had visited.

At the same time, there were escalating tensions between the West and Libyan premier Colonel Gaddafi, who was suspected to have ordered the bombing of a nightclub frequented by US personnel in West Berlin in 1986. Judges in al-Megrahi’s trial conceded the case included ‘a number of uncertainties and qualifications’; yet he was sentenced to life. (Libya later paid $2.7 billion to families of Lockerbie bombing victims, though this was considered a political move rather than an admission of guilt.)

Jim had relocated to Holland for the trial. As the verdict was read out, he fainted, so convinced was he of al-Megrahi’s innocence.

Time has only bolstered his defence of ‘poor’ al-Megrahi, having formed personal relationships with both him and Gaddafi before they died. They would exchange Christmas cards, and when al-Megrahi was given compassionate release in 2009 following a diagnosis of prostate cancer – returning to a hero’s welcome on the tarmac at Tripoli airport – Jim travelled to Libya to see him on his deathbed. At the time, Jim recalled al-Megrahi’s words to him: ‘I am going to a place where I hope soon to see Flora. I will tell her that her father is my friend.’

He was, in Jim’s eyes, only ever an unwitting pawn in geopolitically motivated ‘deception’ that he says is even now preventing justice for Flora and the other victims from being served. He also took a handful of clandestine trips to Gaddafi’s compound (he did not tell any authorities, and only informed Jane imminently beforehand), in which he would hear that the regime had not been to blame. On leaving their first meeting, Jim pinned a UK Families Flight 103 badge to Gaddafi’s lapel as a show of solidarity for the truth. He believes other UK families are onside, although many have never spoken publicly. But there are certainly others, particularly those in the US, who see this affinity with Gaddafi as a grave error.

For Jim, there are two pieces of evidence that point to al-Megrahi’s wrongful conviction. The 2001 case heard that the explosive had first travelled from Malta to Frankfurt, where Flight 103 began its journey to New York. (The London Heathrow stop was a layover.) But Jim believes the bomb was planted at Heathrow. At al-Megrahi’s appeal in 2002, a baggage handler told lawyers that the baggage build-up area at Terminal 3 had been broken into the night before the bombing.

The other piece of evidence relates to the bomb fragments. According to John Ashton, a researcher on al-Megrahi’s legal team, documents not disclosed during the original trial found differences between the metals of the timers being supplied to the Libyans at the time and those within the fragments police recovered from the Lockerbie site. The circuit-board patterns, however, did align, deemed to be the more important evidence.

Clare Connelly of the Lockerbie Trial Briefing Unit also questions the veracity of shopkeeper Tony Gauci’s evidence, as there have been claims that he was paid in connection with his participation in the inquiry, which she says would be ‘totally contrary to the interests of justice’. But in November 2013 the Crown Office said: ‘No witness was offered any inducement by the Crown or the Scottish police before and during the trial and there is no evidence that any other law ­enforcement agency offered such an inducement.’

As for who was actually responsible, Jim argues it was Iran, not Libya. He goes on to suggest that it might have been a retaliatory attack for the US shooting down an Iranian passenger plane, thought to have been incorrectly identified as a fighter jet in July 1988, which killed 290 innocent civilians. In his view, with American hostages held in Iran at the time and an upcoming election, the finger had to be pointed elsewhere. ‘What we’re being told is absolute nonsense from beginning to end. It was designed to protect the relationship between Britain and America and to help in getting home American hostages held by Iranian interests back in ’88.’

Jim insists that the bombmaker was not Mas’ud, as the US alleges, but ‘a Jordanian who was a double agent, or even a triple agent’ – feeding intelligence both to his own country and the CIA, while making explosives for a militant group active in Palestine at the time, called the PFLP-GC. Others have theories of their own around Iran’s involvement: Holt has also said ‘there was a concerted effort, for unexplained reasons, to switch the original investigations away from Iran and the PFLP-GC’ – backing Jim’s belief that the focus on Libya was politically motivated.

For the officials who spent years putting together their case, however, Jim’s theory is not credible enough to upend ‘the biggest case the FBI ever had… I don’t believe, in the history of law enforcement, there was a crime quite like Pan Am 103.’ So says Richard Marquise, who led the FBI investigation. ‘I will never attack [Jim], I will never tell him he’s a liar or wrong. I will never say a negative thing, because I cannot feel his pain; I am sure it’s enormous. But I disagree with his assessment of the evidence.’

The US bereaved take a different view to those in the UK, Marquise says. ‘They would probably tell him he’s wrong. I’ve heard some of them say that. He’s very outspoken. He doesn’t want to believe it.’ Marquise believes that Jim’s meeting the likes of Gaddafi and al-Megrahi, whose early release he considered ‘a slap in the face to the victims’ families’, proved to be effective tools to ‘convince him’ of their innocence.

For Jim, his ‘obsession’ has been an outlet for the pain of losing Flora. As he puts it: ‘It has provided me with a way of coping with my grief.’

As for Jane, she has had little choice but to accept her husband’s dogged pursuit of answers; something Jim is painfully aware of. ‘[I often think] what is it doing to Jane, that I’m still doing this?’ he admits. The Swires have two other children – a son, William, and Cathy, whose birthday is the day after Flora’s. Cathy has found it harder to support her father’s campaign, her mother says. ‘She has never felt as if she could back Jim because it seemed to her it wasn’t relevant, almost – that the loss of a sister and a daughter, that’s what counted, rather than his campaign.’

There is another source of anguish for the Swires – a series of missteps without which Flora may never have boarded Flight 103 in the first place.

In late October 1988, West German police found a bomb hidden inside a Toshiba radio cassette player in an apartment in Neuss, believed to have been manufactured to detonate mid-air. The British Department of Transport (DoT) went on to warn airports and airlines of its existence via telex the next month.

Then, on 5 December, an anonymous threat was phoned in to the US embassy in Helsinki, stipulating that within two weeks, someone would carry a bomb on to a Pan Am flight from Frankfurt to the US. Notices were put up on embassy walls, and US officials were told they could rebook on another flight home for Christmas if they so wished; Interpol informed 147 countries, Britain included – yet the ‘Helsinki warning’ was never made public.

Two days before Lockerbie, a circular featuring images of the explosives authorities feared had been designed to blow up planes was signed by the DoT’s principal aviation security advisor, but never sent out.

Flora had booked her flight relatively last-minute – able to secure a seat on a partially empty plane, just days before Christmas. She, and the other passengers, had no knowledge of the rising threat levels in Western airspace. ‘How do you live with that?’ says Jim. ‘Give me an answer if you’ve got one, because it’s bloody difficult.’

Two emotions permeate our conversations about Flora: fury, when referring to the flight and its aftermath, and a deep pride of his beloved late daughter.

He beams remembering happy times, and pranks she played on him, and mulls with Jane whether Flora had been planning to call on Christmas Day with news of her acceptance into his own alma mater. (Her Cambridge acceptance letter was found by her bedside after her death.) Jane imagines that Flora would have gone on to become a top neurologist, and likes to think that she’d have had a family of her own.

‘How we miss that,’ she says. ‘She would have been a lovely mother and would have had children, and of course that’s been totally denied to all of us.’ The portrait and the lock of hair are solace of a kind, but hardly. ‘What is a lock of hair compared with a living, vibrant, enormously close daughter?’

Jim would like there to be an examination of the evidence in the International Criminal Court. He sees this as the only possible route to justice now – but each passing year makes it less likely.

‘Our numbers are dropping all the time from people dying off from old age,’ he says of the families’ group, ‘and I’m amazed that I haven’t long ago because the stress all this has been over the last 35 years – why I haven’t died of a heart attack, I don’t know… But I would love it if [the truth] were to come out while we were still around.’

John Dower, director of the new documentary, says that his main hope is that those involved in it will ‘get some resolution, some peace, because that’s what struck us most making this, the ongoing trauma. It’s 35 years later, but that trauma is still there.’

Marquise is of a different view: that there is no compelling new evidence to merit a retrial, and that as well as many people who might have been able to contribute now being dead, ‘it’s just too long [ago]... people’s memories fade.

‘I wish we could have done more’ to secure more than one conviction, he adds, but says that he considers the case a professional success. ‘I – with all my heart and soul – honestly believe in the evidence collection, the veracity of the testimony; I believe in the one man who has thus far been convicted. And I will believe that till the day I pass.’

Time has brought more questions, but no more answers. Which is of no comfort to the Swires, who laid Flora to rest in the Isle of Skye, where the family travelled often, her headstone inscribed with the words ‘Longed for always’. Thirty-five years on, nothing has changed.

Lockerbie will be on Sky Documentaries and Now from 25 November

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