On 11 September 2001, four passenger planes were hijacked by radical Islamist terrorists - almost 3,000 people were killed as the aircraft were flown into the World Trade Centre, the Pentagon and a Pennsylvania field. Just hours after the collapse of New York's Twin Towers, a conspiracy theory surfaced online which persists more than 16 years later.
"Is it just me?" an internet user named David Rostcheck wrote, "or did anyone else recognise that it wasn't the airplane impacts that blew up the World Trade Centre?
"I hope other people are actually catching this, but I haven't seen anyone say it yet, so I guess I will. There's no doubt that the planes hit the building and did a lot of damage. But look at the footage - those buildings were demolished," he continued. "To demolish a building, you don't need all that much explosive but it needs to be placed in the correct places... Someone had to have a lot of access to all of both towers and a lot of time to do this. This is pretty grim. The really dire part is - what were the planes for?"
Subsequent investigations made it clear that the tower structures were weakened by the inferno from the planes and felled by the weight of collapsing floors. However even now some people refuse to believe this version of events.
'Something's happened in New York'
On the day of the attacks, Matt Campbell was on holiday in Lanzarote with his wife and two young daughters. He followed the news on television in a state of shock. His brother Geoff, it transpired over the next few agonising hours, was in the World Trade Centre.
"My mother had just flown out the day before to join us for a week," he remembers.
"I recall being on the beach and I think my wife had gone up to one of the restaurants to get some food. She came down saying 'I think something's happened in New York.'
"A few calls here and there and we managed to establish that Geoff was in the North Tower. No-one had heard from him."
Geoff had moved to Manhattan a couple of years earlier. He'd recently got engaged and worked for the Reuters news agency in Midtown. He was on the 106th floor of the North Tower for a conference.
"We immediately thought the worst," says Matt.
"Over the next few days we tried to keep our hopes up. We were stranded - no flights were leaving. All we had was a kind of hazy picture on the hotel's cable TV. The news clips were being repeated showing the plane going into tower two."
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The family got a flight back to the UK after a couple of days and then got onto a flight to New York.
"We went there still clinging to hope that perhaps he had been injured and was unconscious. But we called round a few hospitals and it became clear that there were very few people who were actually injured," Matt says.
Geoff was dead. He was 31. An inquest into his death would not conclude until 2013, though fragments of his shoulder blade were identified among rubble from the World Trade Centre in 2002.
By that point, Matt was beginning to question the official account of what happened. He doesn't subscribe to any one particular conspiracy theory - and online, there are many to choose from - but he's convinced that there is a cover-up which is preventing him from getting answers about his brother's death.
"It probably started at the end of October in 2001," he says. "The more I started to look at stuff over the years, the more things didn't add up."
Matt says he has submitted freedom of information requests to the FBI and other bodies which investigated 9/11.
"It's frustrating. Sometimes they say they're protected from disclosure because they could reasonably be expected to interfere with enforcement proceedings.
"We are 16 years on now. I can't even get basic evidence from the authorities."
He wasn't alone in asking questions. A 2016 study from Chapman University in California, found more than half of Americans believe the government is concealing information about the 9/11 attacks. Sections of the official US government report were redacted for years - and some information is still missing.
However that doesn't mean there's any evidence for the more outlandish online conspiracy theories about the attacks. Some claim the US government was complicit - that officials deliberately let the attacks happen or were even involved in the planning. Experts say part of the reason for the persistence of such conspiracy theories is the dissonance that results when people hear that a relatively small group of men using low-tech weapons caused such cataclysmic carnage.
"It's perfectly natural when something important happens people want to have an explanation," says Professor Karen Douglas, a social psychologist at the University of Kent.
"Often, the official explanation appears quite mundane to people and not particularly satisfying.
"Conspiracy theories often emerge as a result of this need for an explanation that's proportional to the event itself."
And the reinforcing nature of the online world means that the theories have hung around for a decade and a half.
"Information doesn't necessarily spread indiscriminately the way people think it does on the internet and social media. People tend to share it with people who kind of think the same way as they do about these issues in the first place," she says.
The theories have been propelled by several books and films. David Ray Griffin, a professor of religious philosophy and theology, accused the US government of complicity in the attack in his 2004 book The New Pearl Harbour.
The first instalment in filmmaker Dylan Avery's Loose Change series was released in 2005. Vanity Fair suggested the films, which presented many of the most popular 9/11 conspiracy theories, "might be the first internet blockbuster". Millions watched them, sharing the footage on bootleg DVDs, Google Video and internet forums. It was so widely available, a digital copy was even later found in Osama Bin Laden's compound.
In 2006, a California architect called Richard Gage founded Architects and Engineers for 9/11 Truth (AE911 Truth) - a group of engineering and architecture professionals who questioned the official version of events.
The confusion and chaos of 11 September 2001 has also helped the conspiracies find an audience. To take one notable example, a third tower collapsed in Manhattan on 9/11 - WTC7.
This tower has become a key focus for people who question the official account of what happened. A 47-storey building about 100 metres from the twin towers, WTC7 was never struck by an airplane. Two planes plus three towers has equalled plenty of questions - questions compounded by the BBC's own reporting of the collapse of WTC7. In the frenetic, confusing aftermath of the terror attacks, the BBC reported that WTC7 had collapsed twenty minutes before the building actually came down.
In 2007, the BBC's The Editors blog addressed the reports and traced how an on-air comment about an imminent collapse turned into reporting about the building actually falling - shortly before WTC7 did indeed fall.
Any apparent discrepancy was cleared up by a 2008 report by the National Institute of Standards and Technology (NIST) which found that WTC7 collapsed after fires on multiple floors "caused a critical support column to fail, initiating a fire-induced progressive collapse that brought the building down".
But that did not change the minds of the conspiracy theorists.
AE911 Truth board member Roland Angle alleges there are significant errors in the NIST report. His organisation has raised hundreds of thousands of dollars to fund additional research at the University of Alaska Fairbanks. The conclusions of that study will be published in 2018.
"We're trying to clear up the reputation of our own profession," Roland tells me. "We can say what didn't happen that day, no matter what the government report says.
"We think there's a serious issue here."
He called the trial "a bit of a farce" but said: "It's the closest I'm ever going to get to a trial into the murder of my brother.
"There were four other families there. I know one of the ladies there had signed a reinvestigate 9/11 petition back in 2003 or 2004. The other family members, from what I could gather, were pretty much in line with the official narrative.
"But I already expect this. My own experience in England, with family members, is that some people have got past wanting to know what happened. They're still dealing with the never-ending effects of losing a loved-one."
Blog by Chris Bell
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