Image caption 7 World Trade Center has drawn the focus of the 'truthers' who watch video of its collapse and conclude it must have been felled by explosives

An advertising campaign in Canada raises questions about the US government's official account of the terrorist attacks of 11 September 2001. Are Canadians more sympathetic to anti-American conspiracy theories than Americans themselves?

The posters, displayed on buses in Ottawa, show an image of a World Trade Center building collapsing in the inferno of the terrorist attack that morning.

The advertising campaign is supported by a Lafayette, California-based organisation called Architects and Engineers for 9/11 Truth.

The office tower, 7 World Trade Center, may have been destroyed in a controlled demolition, members of the organisation purport, rather than in a fire.

Frank Greening, a nuclear scientist who lives in Hamilton, Ontario, knows the people behind the organisation - truthers, as they are known - well. He first met them years ago.

Like them, he initially thought there was something more to the story of 9/11 than the US government let on. To him it seemed unlikely that a small group such as al-Qaeda could have pulled off such a monumental act of horror.

Canadians are more likely to believe in conspiracies in American life Jordan Michael Smith, Journalist

Besides like many Canadians, he harbours a deep scepticism of US officials and their explanations for why things happen.

"I think Canadians are more tolerant of conspiracy theories when it's anti-American," he said.

Experts agree.

Jonathan Kay, an editor at the National Post and author of Among the Truthers, said 9/11 conspiracy theories resonate for a reason.

"There's an underlying neurosis about American power," he said. "In Canada, it's very fashionable to casually attack the US as neo-imperialist.

"If you went up to someone and said, 'Hey, was George Bush lying about Iraq?' they'd say, 'Oh, yeah, that sounds like something he'd do.'"

Or, as Jordan Michael Smith, a Toronto native who writes for the Wall Street Journal and other publications, explained: "Canadians are more likely to believe in conspiracies in American life. But so is everyone else in the world."

"But belief in conspiracies about life in Canada are extremely rare," he said. "I can't think of any, in fact."

Ted Walter, the manager of the advertising campaign for the 9/11 Truth organisation, said the advertisements have appeared in Ottawa, New York, London and Sydney. In Ottawa, he said, the response was particularly strong.

"The ads in Canada sparked more public discussion than anywhere," he said.

Image caption Truthers said many Canadians were receptive to their argument

The campaign has been controversial. Ottawa councillor Diane Deans, head of the transit commission, said earlier this autumn that she thought the ads seemed "insensitive", according to the Ottawa Citizen.

Last week new ads were approved for the buses, according to Walter. David Pepper, manager of business and operational services, provided the BBC with a statement that said they are undergoing a review of "Advertising Standards". The ads, said Walter, will run until early January.

For an online poll, a sample of Canadians viewed a 30-second video of the collapse of 7 World Trade Center put together by members of the 9-11 Truth organisation.

The participants were then asked whether they were inclined to believe officials who said that a fire caused the tower's collapse or critics who claimed that explosives brought down the building.

Among Canadians, 49% said they were inclined to believe the critics, while 37% of Americans said the same. In other words, the Canadians who responded to the online questions were somewhat more sceptical of the official account than were Americans.

The online poll was unscientific. Yet polls from more established groups also show that Canadians do not necessarily believe US officials, at least when it comes to 9/11.

According to a 2011 Canadian Press/Harris Decima poll reported by the Toronto Star, 42% of Canadians believed that "information about 9/11 is being intentionally hidden", while 47% did not.

Image caption In the US, the truther movement has resisted numerous official, scientific and journalist accounts of what happened that morning

Greening stood out, even among conspiracy-minded Canadians. Indeed, he decided to do his own research. He discovered that al-Qaeda did in fact have the capacity to destroy the World Trade Center.

"When I ran the programme, I thought, 'Holy smoke, it can collapse the building,'" he said. He published his findings in the October 2008 edition of the Journal of Engineering Mechanics.

His research was hardly shocking. It has been backed up by plenty of other experts, including those at Popular Mechanics who published a special report called Debunking the 9/11 Myths.

Yet some Canadians felt he had betrayed the truther cause.

"I was called a government shill - a scumbag," he said. One evening he met some of his critics in a Hamilton coffeehouse. They belonged to "a cell, so to speak, of 9-11 truthers", he said.

"They were polite," he said. "But I could sense this tremendous animosity." He understood.

Image caption Some believe dust from the towers, shown in 2001, contained explosive residue

"They felt they had the moral high ground and this was something that needed to be exposed," he said. "It's very idealistic and I respect their sincerity."

After he published his paper, he began hearing stories that dust from the World Trade Center contained explosive residue. He spoke with a scholar who had previously worked with him and asked if he wanted to go further with the research.

"He said, 'Frank, look, the intent of the paper was to silence the truthers. I consider it mission accomplished,'" Greening recalled.

He was disappointed. Even after proving the truther theories wrong, he still had their idealism. Like conspiracy theorists and truth-seekers in Canada, the US and everywhere else, he believes there is a bigger story that has not yet been told.

"My motive was not to silence anybody, but to get to the truth," he said. "If I ever make it to heaven, my first question will be: 'OK, tell me what really happened on that day.'"