They may never know it, but U.S. air travelers and others set off silent terrorist warning alarms nearly 20,000 times in 2006 when their names matched against the government's centralized terrorist watch list, according to a statistic buried in a Department of Justice document.

The number represents a 27 percent jump over 2005, and points to the growth in the federal Terrorist Screening Center, a joint FBI and DHS operation that controls the government's master list of suspected terrorists. Agencies from the FBI to the NSA nominate names to the database and assign threat level codes to each name. The criteria for inclusion is considered classified.

The list now reportedly includes more than 500,000 names, according to a similar document reported on by ABC News in June. (The Justice Department has since removed that document from its website.)

The State Department queries the list before issuing visas, customs and border agents use it to vet incoming travelers and a subset of the list is exported to airlines for their passenger rating systems. More than 800,000 local and state police can also query the database when they pull over a speeding car or run a detained person's name through their computer system.

Officers who encounter a person on the watch list are put in contact with an employee at the center, who then directs the officer to arrest the person or to try to get valuable details that can be reported back to intelligence agencies.

The Justice Department's proposed budget for 2008 reveals for the first time how often names match against the database, reporting that there were 19,967 "positive matches" in 2006. The TSC had expected to match a far fewer number 14,780. The watch list matched people 5,396 and 15,730 times in 2004 and 2005 respectively.

The report defines a positive match as "one in which an encountered individual is positively matched with an identity in the Terrorist Screening Data Base, or TSDB."

It's not clear from the report whether those numbers include individuals whose names only coincidently match one of those on list, such as when Sen. Ted Kennedy was confused with a former IRA terrorist also named Kennedy.

The watch list has been hounded by these mismatches, which have included small children, former presidential candidates, and Americans with common names such as David Nelson.

The effect of those matches is also unclear, since the FBI won't reveal the number of people who have been arrested or denied visas, boarding passes or entry to the country due to the watch list system.

Airlines use portions of the list to create the "No Fly" list, which dictates who is allowed to get a boarding pass. Persons who are on the government's watch list, but who are not considered a serious threat to aviation, are called "selectees," and have their boarding pass marked with the "SSSS" code that directs screeners to initiate a pat-down search.

Repeated SSSS may indicate that you are on, or match against, a name on the watchlist.

Getting off the terrorist watch list is not a simple procedure, given the government won't confirm if a person is on a list or not, and the TSC doesn't take responsibility for names placed on the list by a law enforcement or intelligence agency.

But a traveler who suspects the watch list is wrongly snagging them at airports or the border can try to use DHS' online redress site, known as DHS TRIP.

The center is asking for a $1.3 million budget increase in 2008 to fund additional employees, including some who will vet names submitted to the agencies. The TSC also wants to station an agent at Chicago's O'Hare airport to help clear up mismatches. New York's JFK, Washington Dulles, Miami and Los Angeles already have such agents.

Currently the TSC employs 108 employees, including 17 FBI agents, with a budget of $103 million.

A TSC spokeswoman said she could not immediately comment on the new statistics.