To Matt Campbell, 9/11 wasn’t just a terrorist attack that killed his brother. It was arguably something far worse: One of the biggest cover ups the world has ever seen.

Campbell believes that what really brought about the destruction of the World Trade Center in 2001 has been kept secret in the 25 years since the tragedy happened.

For the past two decades, he has been on a quest to hold accountable the people who, he believes, have perpetrated this great deception, not just for his brother Geoff, but for all of the nearly 3,000 victims of that day.

Later this week, Campbell, a reflexologist from Brighton, will speak at, ‘Turning the Tide: 9/11 25 years later,’ a conference at the Museo del Barrio in New York where 40 speakers, including first responders who were there on 9/11, will address the audience about their theories. Next month he will attend London’s Supreme Court hearing for 9/11 justice – proceedings for which he fought.

Campbell’s theory? In the face of reams of contradictory evidence from the US government, he believes that incendiary devices – not planes being flown by terrorists – were used to bring down the Twin Towers and World Trade Center 7, which collapsed later in the day.

He isn’t sure who is behind it but suspects the US government and intelligence agencies may have played a role.

If it sounds like a conspiracy theory, then Campbell would disagree. In fact, he resents the term ‘conspiracy theory.’ Calling the word ‘lazy’ he told the Daily Mail that it was ‘usually applied by people who haven’t looked into anything and don’t know the key bits of information out there.’

‘At the end of the day,’ he said, ‘No-one has been held accountable for the deaths in the building and towers, and that’s what we want.’

To Matt Campbell (center), 9/11 wasn’t just a terrorist attack that killed his brother Geoff (right). He believes it was a cover up

Campbell believes that what really brought about the destruction of the World Trade Center in 2001 has been kept secret in the 25 years since the tragedy happened

His brother Geoff, was one of nearly 3,000 victims that day

‘No-one has been held accountable for the deaths in the building and towers, and that’s what we want,’ said Campbell

Campbell is far from the only person who believes there is something more to 9/11 than we know.

Almost as soon as the Twin Towers went down, wild speculation began about what happened and who was responsible.

The attack remains the largest single loss of civilian life in US history and was among the first to have its narrative shaped by the internet, where forums were awash with fringe ideas about who was to blame and fake stories which seemed real.

A cottage industry of books, films and documentaries sprang up rejecting the notion that Osama bin Laden and his terror group Al Qaeda was behind the attacks and claiming to reveal the ‘truth’ about who was to blame.

One prominent theory holds that Mossad, the Israeli intelligence service, was behind the attacks.

Some people believe that the Pentagon was struck by a missile and not by United Flight 93 as the passengers battled the terrorists on board.

But the most prevalent of them all was that the towers came down due to an explosion, not the planes.

Richard Gage is an architect and prominent voice for the unproven theory that the Twin Towers were felled by explosives.

In recent years, Gage has been interviewed and embraced by influential figures like Tucker Carlson, who once ridiculed him but recently changed his mind – a sign of how widespread alternative theories about 9/11 have become.

Gage, told the Daily Mail that 9/11 was likely an ‘inside job by professionals,’ and challenged those who wrote him off as a crank.

‘Are you willing to look at the evidence or do you want to call us names?’ he said.

‘We can’t have a conversation if you just want to call us names.

‘I’m interested in speaking to people who have the courage to look at the evidence and ask themselves: ‘What does this mean?”

That question still drives Campbell in his search for answers.

His brother Geoff who was British but living in New York, was just 31 when he was killed while attending a conference in the World Trade Center for Reuters, the news agency for which he worked.

The North Tower was struck between floors 93 and 99. Geoff’s conference was being held on the 106th floor, trapping him, along with an estimated 1355 others, above the impact zone.

Campbell, now 57, remembered his brother as ‘such a funny, nice guy.’

‘He was very intelligent, he was annoyingly good at anything he tried his hand at,’ he said.

‘He played badminton for England, he was an exceptional chess player.

‘He died too young in an appalling, horrific way.’

Richard Gage is an architect and prominent voice for the unproven theory that the Twin Towers were felled by explosives

The most prevalent theory is that the towers came down due to an explosion, not the planes

Campbell, now 57, remembered his brother Geoff (right with their brother Rob) as ‘such a funny, nice guy’

The attack remains the largest single loss of civilian life in US history

Campbell began to ask questions about the official version of 9/11 after reading news articles calling the war in Afghanistan a ‘farce.’

He then devoured books about the disaster – more than 160 titles – first on intelligence failures and then other subjects.

‘Like most things, the more I looked it almost seemed so far from what we were told in the first few days of 9/11,’ Campbell said.

‘As I started to look, I got frustrated that nobody was being held accountable.’

To many people, some of Campbell’s frustrations will seem well founded.

Despite 25 years passing since 9/11, the alleged mastermind of the attack, Khalid Sheikh Mohammed, has yet to go on trial at a military court in Guantanamo Bay, the US base in Cuba, where he has been held for two decades.

Last month, a judge ruled that the alleged terrorist’s confessions cannot be used in the secretive trial in a military court because they were obtained under torture.

A judge ruled last year that a long-running lawsuit by 9/11 families against the government of Saudi Arabia for allegedly helping the plane hijackers can move ahead.

But Campbell expects that case to be settled meaning there could well be no proper court hearing or trial of any kind.

He also said it was, ‘disgusting’ that the final report of the 9/11 Commission, which was set up by the US government to investigate how the attack happened, barely discussed how the Twin Towers went down.

The inquest into Geoff’s death in 2013, along with 10 other British victims, lasted just 90 minutes. Campbell is pushing to have it reopened. A hearing is scheduled for October at the UK’s Supreme Court. The Attorney General has previously blocked any such renewed inquest; October’s hearing will center on whether the office has that power without court oversight.

On other points, Campbell strays a long way from the official narrative from the National Institute of Standards and Technology (NIST), an arm of the Commerce Department which was tasked with explaining what happened.

NIST’s exhaustive investigation involved more than 200 professionals and 400 emergency responders and interviews with 1,056 survivors from the World Trade Center buildings.

The conclusion was that the impact of the planes caused the Twin Towers to collapse and debris from those buildings caused fires that destroyed WTC 7.

Despite 25 years passing since 9/11, the alleged mastermind of the attack, Khalid Sheikh Mohammed, has yet to go on trial

Thee final report of the 9/11 Commission was set up by the US government to investigate how the attack happened

World Trade Center 7 collapsed later in the day

NIST’s conclusion was debris from the Twin Towers caused fires that destroyed WTC 7

Reached this week, the NIST declined to make anyone available for comment and directed the Daily Mail to the published reports.

‘We will let those materials speak for themselves,’ a spokesman said.

Campbell’s view is that the NIST report is deeply flawed.

He said: ‘It really was Building 7 for me, it just seemed so improbable that was how NIST described it, how it was normal office fires when the building fell at freefall acceleration for 2.5 seconds.

‘I started looking into the Towers and they all have strange anomalies.

‘There was a lot of evidence supporting in terms of eyewitnesses and expert testimony relating to the use of explosives or incendiaries, something other than jet fuel.’

Gage has a similar theory fueled by the fact that the steel columns inside the towers were melted.

‘Steel is not melted by jet fuel,’ Gage said. ‘The only thing that can do this is incendiaries.’

The NIST report said this was inaccurate.

The report said that fires caused the steel beams of WTC 7 to expand and caused a ‘key structural column to fail.’

‘The failure of this structural column then initiated a fire-induced progressive collapse of the entire building,’ the report said.

The report added that WTC 7 did not collapse in a free fall, stating: ‘NIST concluded that blast events inside the building did not occur and found no evidence supporting the existence of a blast event.’

Nor was there any evidence of incendiary devices in the Twin Towers, the report said.

So, if it wasn’t Al Qaeda, then who could have carried out the attack, and why?

‘I don’t know who,’ Campbell said.

‘But I do know the US authorities have made it extremely hard to get all the evidence regarding 9/11 released unredacted to the public.

‘That told me they’re to some extent involved in the cover up. For whom I don’t know.

‘You have to look at those doing the protecting, why are they doing it and who are they protecting.’

People deal with grief differently, and some may think that Campbell – quite understandably – has gone down the wrong path after suffering an appalling tragedy.

The report said that fires caused the steel beams of WTC 7 to expand and caused a ‘key structural column to fail’

Campbell’s view is that the NIST report is deeply flawed. He said: ‘It really was Building 7 for me, it just seemed so improbable’

The attack was among the first to have its narrative shaped by the internet, where forums were awash with fringe ideas about who was to blame and fake stories which seemed real

Campbell, along with his mother Maureen and brother Rob, are visiting New York to remember Geoff

But he doesn’t see it that way.

‘How can it be the wrong path if it’s for truth and justice?’ said Campbell.

‘Try telling that to the Bloody Sunday families,’ referring to the infamous massacre of 25 civilians by British soldiers in 1979 in Ireland that led to a damning public inquiry.

‘I don’t subscribe to the ‘bring closure’ stuff either,’ Campbell said.

‘There is evidence which has not been put before a court regarding my brother’s death. All I ask is that it is allowed to be.’

This week, Campbell will be in New York along with his mother and other brother, Rob, for the first time since 9/11.

The visit marks an emotional milestone in Campbell’s campaign for the truth, but he will never give up looking for what he sees as the truth.

‘This (9/11) will be debated forever,’ he said. ‘But I want a trial, I want evidence to be tested, that’s what you expect after a murder – let alone a mass murder.’