CLARKSBURG, W. Va. - The FBI is embarking on a $1 billion effort to build the world's largest computer database of individuals' physical characteristics, a project that would give the government unprecedented abilities to identify people in the United States and abroad.

Digital images of faces, fingerprints and palm patterns are already flowing into FBI systems here. Next month, the FBI intends to award a 10-year contract that would significantly expand the amount and kinds of biometric information it receives.

In the coming years, law-enforcement authorities around the world will be able to rely on iris patterns, face-shape data, scars - perhaps even the unique ways people walk and talk - to solve crimes and identify criminals and terrorists.

The FBI will also retain, on an employer's request, fingerprints of employees who have undergone criminal-background checks so the employer can be notified if employees have a brush with the law.

"Bigger. Faster. Better. That's the bottom line," said Thomas E. Bush III, assistant director of the FBI's Criminal Justice Information Services Division, or CJIS, which operates the database from its headquarters in the Appalachian foothills.

The increasing use of biometrics for identification is raising questions about the ability of Americans to avoid unwanted scrutiny. It is drawing criticism from those who worry that people's bodies will become de facto national identification cards. Critics say such government initiatives should not proceed without proof that the technology really can pick a criminal out of a crowd.

The use of biometric data is increasing throughout the government. For two years, the Defense Department has been storing images in a database of fingerprints, irises and faces of more than 1.5 million Iraqi and Afghan detainees, Iraqi citizens, and foreigners who need access to U.S. military bases. The Pentagon collects DNA samples from some Iraqi detainees, which are stored separately.

The Homeland Security Department has been using iris scans at some airports to verify the identities of travelers who have passed background checks and who want to move through lines quickly. It is also looking to apply iris- and face-recognition techniques to other programs.

Homeland Security already has a database of millions of fingerprints, including records collected from U.S. and foreign travelers stopped at borders for criminal violations, from U.S. citizens adopting children overseas, and from visa applicants abroad.

"It's enabling the Always on Surveillance Society," said Barry Steinhardt, director of the Technology and Liberty Project of the American Civil Liberties Union.

If successful, the system planned by the FBI, called Next Generation Identification, will collect a wide variety of biometric information in one place for identification and forensic purposes.

In an underground facility the size of two football fields, a request reaches an FBI server every second from somewhere in the United States or Canada, comparing a set of digital fingerprints against the FBI's database of 55 million sets. A possible match is made - or ruled out - as often as 100,000 times a day.

Soon, the server at CJIS headquarters will also compare palm prints and, eventually, iris images and face-shape data.

If all goes as planned, a police officer making a traffic stop or a border agent at the airport could run a 10-fingerprint check on a suspect, and know within seconds if the person is on a database of the most wanted criminals and terrorists. Intelligence agents could exchange biometric information worldwide.

Advocates say bringing together information from a wide variety of sources and making it available to multiple agencies increase the chances of catching criminals.

Skeptics say such projects are proceeding before there is evidence that they reliably match suspects against a huge database.

In the world's first large-scale, scientific study on how well face recognition works in a crowd, the German government found this year that the technology, while promising, was not yet effective enough to allow its use by police.

The study, conducted from October 2006 through January at a train station in Mainz, Germany, found that the technology could match travelers' faces against a database of volunteers more than 60 percent of the time during the day, when the lighting was best. But the rate fell to 10 percent to 20 percent at night.

Accuracy improves as techniques are combined, said Kimberly Del Greco, the FBI's biometric-services section chief. The Next Generation database is intended to "fuse" fingerprint-, face-, iris- and palm-matching capabilities by 2013, she said.

To safeguard privacy, audit trails are kept on everyone with access to a record in the fingerprint database, Del Greco said, and people may request copies of their records.

Privacy advocates worry about the ability of people to correct false information.

"Unlike, say, a credit-card number, biometric data is forever," said Paul Saffo, a Silicon Valley technology forecaster. He said he feared that the FBI, whose technology record has been marred by expensive failures, could not guarantee the data's security.

"If someone steals and spoofs your iris image," Saffo said, "you can't just get a new eyeball."