Federal data show the rise comes despite the removal of 33,000 entries last year by the FBI's Terrorist Screening Center in an effort to purge the list of outdated information and remove people cleared in investigations.

It's unclear how many individuals those 33,000 records represent — the center often uses multiple entries, or "identities," for a person to reflect variances in name spellings or other identifying information. The remaining million entries represent about 400,000 individuals, according to the center.

The new figures were provided by the screening center and the Office of the Director of National Intelligence in response to requests from USA TODAY.

"We're continually trying to improve the quality of the information," says Timothy Edgar, a civil liberties officer at the intelligence director's office. "It's always going to be a work in progress."

People put on the watch list by intelligence and law enforcement agencies can be blocked from flying, stopped at borders or subjected to other scrutiny. About 95% of the people on the list are foreigners, the FBI says, but it's a source of frequent complaints from U.S. travelers.

In the past two years, 51,000 people have filed "redress" requests claiming they were wrongly included on the watch list, according to the Department of Homeland Security. In the vast majority of cases reviewed so far, it has turned out that the petitioners were not actually on the list, with most having been misidentified at airports because their names resembled others on it.

There have been 830 redress requests since 2005 where the person was, in fact, confirmed to be on the watch list, and further review by the screening center led to the removal of 150, or 18% of them.

Without specific rules for who goes on the list, it's too bloated to be effective, says Tim Sparapani, a lawyer with the American Civil Liberties Union.

A 2007 audit by the Government Accountability Office said more needed to be done to ensure the list's accuracy, but still found that it has "enhanced the U.S. government's counterterrorism efforts."