The FBI has reportedly sent a bulletin to state and local law enforcement identifying as possible terrorists people who harbor conspiracy theories about the 9/11 attacks. Various polls over the years have shown that large portions of the United States population think that the government is hiding something about the 9/11 attacks, or that the Bush administration or its CIA knew about them in advance and didn't stop them.
A November 2007 Scripps Howard poll, for example, asked people how likely they thought it was that "some people in the federal government had specific warnings of the 9/11 attacks in New York and Washington, but chose to ignore those warnings." 62% of respondents said "very" or "somewhat" likely. A 2007 Rasmussen poll found that 29% of people surveyed thought the CIA knew about the attacks in advance.
Therefore the FBI is instructing state and local police officers to treat as potential terrorists large numbers of Americans, simply because they hold views that people in polite company might consider unorthodox but are actually fairly mainstream.
The FBI has access to lots of the data the NSA collects about all of us, without warrants, as well as formidable intelligence collection schemes and powers of its own. How likely is it that visiting a 9/11 Truth website will land you on some kind of list of dangerous dissidents? If holding a belief that the CIA arranged 9/11 is a potential terrorist indicator, could the FBI use evidence that you visit Truther websites and watch Truther videos as cause to put you on a no-fly list, or target you for specialized surveillance?
Who knows. But if the FBI really thinks that people who believe the US government did 9/11 fit some kind of terrorist profile, the Bureau should watch out for Turkey and Mexico. A 2008 poll showed that large numbers of people from those countries thought the US attacked itself in September 2001.
The FBI's bulletin to local cops also advises them to watch out for people who harbor "fury" at the United States government for its actions abroad. Presumably anyone who visits Amazon or Powell's online to look for Chalmers Johnson's "Blowback: The Costs and Consequences of American Empire" could be added to a Domestic Terror list, as well.
One of the hallmarks of totalitarian societies is an emphasis on political conformity. Another is the equation of dissent with subversion, treason, or terrorism. Let's hope local police don't bite any of this poisoned fruit.