Around the clock, government computers and forensic examiners compare fingerprints the U.S. government and its partners in other countries have collected to the prints of foreign nationals entering the United States.
Military and intelligence officials have been secretly taking thousands of fingerprints from terrorist safe houses, training camps, bomb factories and battlefields in Iraq and Afghanistan.
The war zones, the caves in Afghanistan and other places where potential terrorists hide out can be sources of fingerprints, said Robert Mocny, director of the Department of Homeland Security's U.S. Visitor and Immigrant Status Indicator Technology Program — also known as US-VISIT.
"They will have used a laptop," he said. "They may have used another device. We may pick up fingerprints from a bomb fragment."
The fingerprints of suspects in the various London plots in recent years have been placed in the national database, and U.S. officials expect any fingerprint taken in the most recent London attack will be added.
The goal is to catch a terrorist — who has not made it onto a watch list — trying to sneak into the country to launch an attack.
"What we really want to do is look for the unknown terrorist," said Homeland Security Secretary Michael Chertoff. "We can run the prints of visitors against those databases and identify people who have trained in camps or built bombs whose names we may not yet know."
A secret facility outside Washington, D.C., houses a database containing more than 60 million fingerprints.
It stores the prints of not only suspected terrorists but fugitives and illegal immigrants who have been kicked out of the country.
At ports of entry, such as airports, officials scan the fingerprints of every foreign national entering the United States as part of an electronic dragnet.
The effort aims to track individuals who might have created a false identity to get around security.
"We are looking for people who are trying to beat the system," Mocny said. "The fingerprint doesn't lie. The name and date of birth can be changed but the fingerprint cannot."
So far, more than 1,300 criminals and illegal immigrants have been caught using bogus identities.
Homeland security officials plan to expand the program, making the data available to 650,000 police installations across the country.
Beginning this fall, foreign nationals will have to submit their 10 fingers for printing, not just two. The government believes this will give it more chances to catch a terrorist.