For some Americans, the deaths of nearly 3,000 people was not the scariest thing about 9/11. It was realizing who carried out the attacks: yes, the American government.
Defying countless official and non-governmental enquiries, media reporting, and often common sense, a significant number of people fervently disbelieve that Osama bin Laden's al-Qaeda sent 19 hijackers to execute the September 11, 2001 massacre.
Instead, elements within George W. Bush's administration -- or perhaps Israeli agents -- used pre-placed explosives and missiles to blow up the World Trade Center and Pentagon.
A relatively milder version is that the US government didn't actually blow up its citizens, but knew full well the attacks were coming, then did nothing to stop them.
In both cases, the reasoning is that Bush's team sought to justify the invasions of Iraq and Afghanistan, as well as a domestic clampdown on civil liberties.
Wild as they might sound, the conspiracy theorists are not just some tiny fringe.
A 2006 Scripps Howard poll found 36 percent of Americans believed in some form of government 9/11 conspiracy. Other polls have found widespread support not only in the Arab world, but also in France, where a book called "Horrifying Fraud" sold 200,000 copies shortly after 9/11.
Even a decade later, the conspiracy movement is alive and kicking in the United States.
Groups such as Scholars for 9/11 Truth and Justice, or Architects and Engineers for 9/11 Truth, see themselves as serious researchers exposing perhaps the biggest cover-up in US history.
In fact the real loonies, says David Ray Griffin, author of "The New Pearl Harbor" and "Cognitive Infiltration," are those who believe the government version about the World Trade Center and Pentagon.
"If we define miracles as a violation of scientific principles, in particular the principles of physics and chemistry, there are about a dozen miracles in the official story," he told KPFA, a liberal radio station in California.
A summary of the leading theories is contained in "Loose Change," a homemade documentary viewed nearly 125 million times on Google and some 30 million worldwide on YouTube, according to director Dylan Avery.
Splicing news footage and interviews, all set to a catchy music beat, "Loose Change" collates and expands on all the urban legends about 9/11.
-- The Twin Towers could not have collapsed from airplane impact alone.
-- The astonishingly rapid collapse of World Trade Center tower 7, despite not being hit by a plane, bore the hallmarks of professional demolition.
-- Wall Street trading on stocks directly affected by the disaster showed some people knew in advance about what was coming.
-- A US missile, not American Airlines Flight 77, smashed into the Pentagon.
-- United Airlines Flight 93 did not crash in a Pennsylvania field, but vanished, perhaps after being shot down by a fighter jet.
The United States has long been fertile ground for conspiracy theories.
In this parallel world, John F. Kennedy was assassinated by anyone ranging from the CIA to Cuban exiles. The Moon landing pictures were taken in a studio, while the US government is covering up evidence of UFOs. And American GIs are still held in bamboo cages in Vietnam.
The most recent addition is the "birther" movement, which claims Barack Obama was not born on US soil -- and therefore is an illegitimate president.
The conspiracy theorists remain a minority and face the scorn of the counter-conspiracy movement on websites like www.debunking911.com and www.screwloosechange.blogspot.com, which claims to prove wrong almost every statement made during the film.
But Kathy Olmsted, who teaches history at University of California Davis, said extreme mistrust of government is understandable.
After all, the Bush administration spent huge energy promoting its own false conspiracy theories about Saddam Hussein, claiming that the Iraqi leader possessed weapons of mass destruction and insinuating that he was linked to 9/11.
"The Bush administration did certainly twist the truth, if not outright lie during the Iraq war," Olmsted said. "So people say, 'how do we know the truth about 9/11'?"
Conspiracy theories may also aid people still struggling to comprehend the shock of what happened ten years ago.
"It was difficult for people to believe that 19 men armed with box cutters could cause such damage and kill so many people," said Professor Rich Hanley, who lectures on the media and popular culture at Quinnipiac University.
"It plays to how shocking this was to the American psyche."