DALLAS — Despite years of investigations, there are thousands of people who don't believe the official version of how the World Trade Center collapsed, who was responsible or what the government knew and when.

"Before 9/11, I was a working-class person, going through life, pretty much accepting everything given and told to me," said Bryan Black, a 50-year-old carpenter from Commerce, Texas, at a recent meeting of the North Texans for 9/11 Truth at Barbec's Restaurant in Dallas. "I'm starting to see things. I'm more open to skeptical conversation."

The group has 50 active members; 200 on the mailing list.

The skeptics — they prefer the term "9/11 truth activists" instead of "truthers" — have persisted, even thrived in the decade since 2001, with proponents from former Alaska Sen. Mike Gravel to comedian Rosie O'Donnell. And they have mobilized with lightning speed on the Internet, with YouTube videos of the trade center collapsing again and again.

"There's really a foundation of reality here," said Ted Walter, who has worked unsuccessfully to prod New York City officials into reopening an investigation of how 7 World Trade Center collapsed on the afternoon of Sept 11. "We believe that if all of the American public saw footage of building 7 on the nightly news, it would lead to widespread skepticism of 9/11."

There's no real estimate of the numbers of people in the 9/11 "truth" movements — there's no one leader of the skeptics. A group called Remember Building 7 presented New York's City Council with a petition in 2009 signed by 80,000 people calling for an independent probe into the attacks. Other groups include Architects & Engineers for 9/11 Truth, Scholars for 9/11 Truth and the 9/11 Commission Campaign, founded by Gravel.

The "truthers" generally have about a dozen beliefs surrounding what happened on that day, although there are some variations on who was responsible for the attacks and why:

• Explosives brought down the World Trade Center, not hijacked jetliners.

• There were warnings of the impending attacks from 11 different countries, and fighter jets could have intercepted at least one of the four planes that day.

• Criminal conspiracies within the government caused the attacks.

The National Institute of Standards and Technology (NIST) conducted a probe that took six years to complete of the tower collapses; the last report found that fire caused the collapse of 7 World Trade Center, a skyscraper north of the twin towers. In the collapses of the twin towers, the agency found that extreme heat from the jetliner crashes caused some steel beams to lose strength, causing further failures in the building until the entire structure succumbed.

The investigation "was the most comprehensive examination of a structural failure ever conducted," said Shyam Sunder, lead investigator of the collapse investigation and led to 40 building-code changes to make safer, terror-proof skyscrapers. NIST maintains a website with its reports and computer-based animations that reconstruct its findings to reach out to the public.

For Bob McIlvaine's son, it was the injuries found on his son Bobby's head, arm and skin that made him think the hijacked jetliner and building collapse couldn't have done it. He believes that explosives were detonated in the towers' basement before the planes hit the towers.

McIlvaine has not been able to determine where his son was when he died, but from the injuries — which include skin that was burned post-mortem — he assumes that his son was in or near the tower's lobby. McIlvaine questions the government's explanation that a fireball came down through the elevator shafts and burned those in the lobby.

"I spend three hours a day, every day, doing research on 9/11," McIlvaine said. "To me, this was a murder investigation. My son was murdered."

Tom Theimer watched the World Trade Center crumble while drinking coffee and watching television in his suburban Dallas home. Shaken, he bought flags for his porch and bumper stickers for his car reading "We will never forget."

A few years later, a friend of Theimer's wife casually mentioned that 9/11 "was an inside job." Theimer was livid and turned to the Internet, to prove the friend wrong.

The websites, the books and the documentaries he saw online convinced him. He was wrong, and so was the system.

"I was duped," Theimer said. "It really hurt. I cried. I couldn't sleep for months."

Theimer said that he and others in Dallas are planning to show a new 9/11 documentary on the 10th anniversary. Remember Building 7 is trying to raise $1 million by Sept. 11 to support a new investigation into the collapses.

A conference on alternate 9/11 theories is being held in Toronto on Sept. 11.

The conference is headed by the International Center for 9/11 Studies, which was founded by James Gourley, a 31-year-old Dallas-area attorney who began to question the events of Sept. 11 during law school, while watching an activist make his argument on C-Span.

Gourley is aware of the theories about how skeptics are simply trying to justify and explain a random, horrific event.

"It's basically a backwards way of saying we're psychologically deranged," he said. "It's questioning the psychology of the people instead of questioning the facts."

Reported by The Associated Press writer Tamara Lush, who is traveling the country writing about the 10th anniversary of the 9/11 attacks. Follow her on Twitter at http://twitter.com/tamaralush.