I'd be interested to know what Homeland Security knows about those neo-Nazi and other white supremacist groups. Are they tracking them? Are any of them on no-fly lists?
Their members probably are not being tracked and, in general, it wouldn't be possible for the government to place them on no-fly lists. U.S. law enforcement agencies—both federal and state—can only begin investigations if they have "probable cause" that a crime has been committed or is about to be, at which point they can obtain judicial warrants to conduct searches, but the "about to be" part requires evidence that is difficult to obtain without an informant who is participating in an active plan to commit a crime. Political speech and belief, per se, are constitutionally protected no matter how extreme, and do not constitute probable cause to trigger law enforcement investigations.
Unlike most countries, the United States has no domestic intelligence service that can monitor individuals who are suspected to be likely to commit crimes but have not yet done so. There have been proposals over the years to give the FBI limited intelligence authority similar to that exercised by the domestic intelligence agencies of some European democracies—perhaps not as extensive as the British MI-5, but something similar to the powers of the German BfV—however Congress has never acted on them. (The FBI does have
counter-intelligence jurisdiction: the ability to monitor individuals who are believed to be spying for foreign governments, whether they are foreigners, or citizens or permanent residents of the United States.)
Most terrorist plots that have been detected involve individuals with foreign connections, and, as the whole world is aware, the United States has a number of agencies that gather foreign intelligence, as well as information-sharing arrangements with the intelligence agencies of numerous other governments. Based on the press accounts, it apparently is through involvement with foreign collaborators that most plans to commit terrorist acts here have been discovered. But domestic groups, even if they are actively planning violence, probably find it fairly easy to fly under the law enforcement radar.