The documents were obtained by the Electronic Privacy Information Center, which gave them to The New York Times. One memo shows people can remain on the list after terrorism-related charges are dropped or they are acquitted.
"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 memorandum sent in December to FBI field offices said.
The watch list includes 420,000 individuals, 8,000 of them U.S. citizens. A smaller group, 500 U.S. citizens, are among the 16,000 people on the "no-fly" list.
Timothy J. Healy, the terrorism center's director, said the documents show procedures for adding and removing names are not "haphazard."
"There has been a lot of criticism about the watch list," he said. "But what this illustrates is that there is a very detailed process that the F.B.I. follows in terms of nominations of watch-listed people."