Dallas ? In Dealey Plaza, with the white “X” painted on the spot where President Kennedy was assassinated, ask anyone about the grassy knoll and the second gunman.

Conspiracy theories come with the territory here. And at Barbec’s Restaurant on the other side of this sprawling city, six men sit on a covered porch and convene a meeting of the North Texans for 9/11 Truth group and talk about the government’s lies about 9/11.

The group has 50 active members; 200 on the mailing list. And they number among many thousands who, after years of investigations, don’t believe the official version of how the World Trade Center collapsed, who was responsible or what the government knew and when.

Politics doesn’t have anything to do with it; two were once staunch, Bush-voting conservatives; two are progressives and two weren’t even interested in current events until after the 2001 attacks.

“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, “I’m starting to see things. I’m more open to skeptical conversation.”

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 unlike the years that Kennedy assassination conspiracies took to develop, they have mobilized with lightning-like 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.”

For many, conspiracy theories aren’t terrifying; they’re more comforting than the idea that an event as terrifying as Sept. 11 could be so — random.

Conspiracies can be a “security blanket” for explaining away the horrific, asserts Patrick Leman, a University of London professor who researches 9/11 theories. “It stops us from having to confront the unpredictability of life.”

Jonathan Kay, a columnist with the Canadian newspaper The National Post and the author of a book about conspiracy theories, said it’s normal for people to seek out complicated and detailed explanations of big events.

“There is something in the human mind that rebels against the idea of random forces or individuals being able to bring down powerful people or powerful icons,” said Kay.

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 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.

Sunder acknowledges it hasn’t reached everyone.