Washington (CNN) -- Newly released FBI documents say a person's name could be on the U.S. government's terror watch list even if terror charges have been dismissed or if the person has been acquitted in a trial.
A December 2010 FBI memo distributed to the bureau's field offices leaves the door open for a person to remain on the list following acquittal or dismissal of charges, although it says such a person "must still meet the reasonable suspicion standard in order to remain on, or be subsequently nominated to, the terrorist watch list."
The document says in most cases a person's name would be removed from the roster.
The memo says a person cannot be watch-listed based solely on "hunches" and that there must be "particularized derogatory information" that a person could be a national security threat. In addition, the memo says there must be at least "one source of corroboration" connecting someone to terrorist activities.
The memo was among 91 pages of documents the FBI released in response to a Freedom of Information Act request by the Electronic Privacy Information Center. Many details about how the watch list works have not been known to the public. Inclusion on the roster could mean someone would not be able to fly on an airplane or cross a border to enter the United States. At a minimum, a person could face additional screening or questioning.
Among the information revealed in the documents is the guidance given to law enforcement officers about what action they should take if they come into contact with someone on the watch list. It details a three-tiered system of "handling codes" in which a person would be arrested if there is a outstanding arrest warrant; questioned while checks are made with the Department of Homeland Security and other officials about whether someone should be detained; or allowed to move on without interference.
The December FBI memo repeatedly states law enforcement agents should not tell a person that his or her name may be on the terror watch list.
Timothy Healy, director of the FBI's Terrorist Screening Center, said the newly released documents show the watch list process is "very detailed and exhaustive, and provides a mechanism for modifying and removing individuals from the watch list based on the latest intelligence."
In a written statement, Healy called the watch list "one of the most effective tools we have in the ongoing fight against terrorism." He also said most of the people who have filed complaints are mistaken in believing they have been listed. Healy said less than 1% of those who filed complaints were on the terror watch list.
Ginger McCall, a counsel for the Electronic Privacy Information Center, expressed concern about the information in the FBI documents. "In the U.S., you are supposed to be presumed innocent, but not on the watch list," McCall told CNN. "If you are acquitted you can still be on the list."
She also criticized the instructions about not telling people whether they are on the roster of possible terrorists. "It's frightening to think you could be on such a list and have no ability to find out," McCall said.
Approximately 420,000 people are included on the government's terrorist watch list, according to information released just before the 10th anniversary of the September 11 terror attacks. Around 16,000 people are on the no-fly list and fewer than 500 of those are U.S. citizens.