The president’s top counterterrorism adviser has the temporary ability to prevent categories of people — already in the government's vast databases of known or suspected terrorists — from flying, in the face of a credible threat, according to a newly published report.

The previously unknown authority allows the adviser to add groups of names to the no-fly list for up to 72 hours. After that, the president’s Cabinet secretaries or their deputies have to extend the action.

If they don’t concur, the names are removed from the no-fly list.

The government added about 1.5 million names to its terrorist watch lists during the past five years, according to court documents. The no-fly list is more selective and is believed to hold several thousand names.

The unclassified information was contained in a 166-page document entitled the “March 2013 Watchlisting Guidance” that was published Wednesday by the online magazine The Intercept.

The document describes the procedures for placing people on a watch list, noting that nominations are based on a “reasonable suspicion that the individual is a known or suspected terrorist.”

The guidance defines a suspected terrorist as an individual “known or reasonably suspected to be or have been engaged in conduct constituting, in preparation for, in aid of, or related to terrorism or terrorism activities.”

Hina Shamsi, who directs the American Civil Liberties Union’s National Security Project, said the criteria for the watch list was “shockingly vague and broad, riddled with exceptions and will inevitably ensnare innocent people.”

An official at the National Counterterrorism Center defended the system.

“The watch-listing system is an important part of our layered defense to protect the United States against future terrorist attacks,” said the official, who spoke on the condition of anonymity to discuss intelligence matters. “Before an American may be included on a watchlist, additional layers of scrutiny are applied to ensure that the listing is appropriate.”

The official also said elevating categories of individuals to the no-fly list was “very rarely used” and “applied to people who are already in the broader category of known or suspected terrorists.”

The document lists material that can be collected by officials who encounter known or suspected terrorists, including “Titles of books, DVD/CD, brochures being carried and their condition (e.g., new, dog-eared, annotated, unopened, professional journals).”

There is also guidance on former detainees at the U.S. military prison at Guantanamo Bay, Cuba. They are automatically placed on the no-fly list unless the “president has specifically written to Congress that the released detainee poses ‘no threat to the United States, its citizens, or its allies.’ ”