The 91 pages of newly disclosed files include a December 2010 guidance memorandum to F.B.I. field offices showing that even a not-guilty verdict may not always be enough to get someone off the list, if agents maintain they still have “reasonable suspicion” that the person might have ties to terrorism.

“If an individual is acquitted or charges are dismissed for a crime related to terrorism, the individual must still meet the reasonable suspicion standard in order to remain on, or be subsequently nominated to, the terrorist watch list,” the once-classified memorandum says.

Ginger McCall, a counsel at the Electronic Privacy Information Center, said: “In the United States, you are supposed to be assumed innocent. But on the watch list, you may be assumed guilty, even after the court dismisses your case.”

But Stewart Baker, a former Homeland Security official in the Bush administration, argued that even if the intelligence about someone’s possible terrorism ties fell short of the courtroom standard of “beyond a reasonable doubt,” it could still be appropriate to keep the person on the watch list as having attracted suspicion.