Government contractors hope a mix of new and existing technologies will better identify foreigners entering the United States through thousands of miles of land borders, without causing backups that stretch halfway to the ocean.

One key ingredient is a rapidly emerging but controversial technology known as RFID, or radio frequency identification, which companies are increasingly using to remotely track products. Border stations would use RFID-equipped identity cards to verify fingerprint information, but experts say the initiative faces daunting challenges, from cost to concerns about enforcement.

On Tuesday, the Department of Homeland Security announced that Reston-based Accenture LLP won a contract worth as much as $10 billion to add border crossings to the government's five-month-old program of fingerprinting and photographing visitors who arrive at airports and seaports.

By extending the program to land border stations, the agency aims to create a database of visitors that can help U.S. authorities keep out, detain or possibly monitor suspected terrorists or criminals.

Under the system used at U.S. airports and seaports since the beginning of the year, visitors have their fingers scanned to match fingerprints given when they applied for their visas to enter the country. U.S. Customs officers can use that opportunity to check the visitor against watch lists or other databases.

But such checks would slow border crossings by land to an unacceptable crawl. In 2002, land crossing stations processed 238 million non-U.S. citizens.

Large amounts of U.S. business and trade are dependent on tens of thousands of daily crossings at the Mexican border. Nearly 7 million Mexican nationals -- workers, deliverymen or business owners -- have what are called border crossing cards, which allow them entry into the United States for 72 hours and restrict them to certain distances from the border.

These visitors make an average of 285,000 crossings a day. For them, the Accenture team plans to create a version of the E-ZPass toll-booth system, which employs RFID technology on highways including Interstate 95. A holder of an E-ZPass device can zip through designated toll-booth lanes, where electronic scanners "read" the pass and automatically deduct the cost of the toll from the user's account.

Similarly, adding RFID would allow crossing-card holders to move through border checkpoints more quickly than other visa holders, who would be subject to the same fingerprint and photo scans used at airports. Crossing-card holders account for a little less than half of all crossings a year.

The existing crossing cards are issued by the State Department and include fingerprint data. But typically, crossing guards don't require a fingerprint scan to match to the card; they simply see the photo that pops up on a monitor and make a visual match with the card holder.

Although the details of the RFID program are yet to be worked out, government and Accenture officials said the goal is to develop a card that could be automatically scanned to verify the fingerprint data as well.

In one scenario, the cardholder might need to place his or her finger on a certain spot on the card, which would activate the card so its data could be read by the scanner as the cardholder crossed the border. The card would use a radio signal, and an RFID tag does not require a direct line of sight to be read, as do the bar codes on consumer products.

"In theory, you can transmit biometric information using RFID . . . and you could fly by at 40 miles an hour, and it could be read" by a remote scanner, said Department of Homeland Security spokesman Dennis Murphy. That won't be allowed, of course, because a car might contain several people, all of whom must be checked.

Eric Stange, managing partner for defense and homeland security at Accenture, said RFID technology has "a lot of promise" in not only allowing fast verification, but in allowing information on the card to be shared with other databases. Stange said his firm, which will use several other technology companies as subcontractors, plans to build a system that would allow for technology enhancements, such as going beyond photographs to retina or iris scans.

"Conceptually, the idea is a very good thing," said K. Jack Riley, director of the homeland security program at Rand Corp. "But the land border is going to be much more difficult than the constrained environment of airports."

Riley wonders what will happen when the new system simply pushes terrorists or criminals to stop crossing at official outposts and make use of the hundreds of miles of unprotected border. Additionally, the program will operate at only 50 of the 165 border crossing stations at the end of 2004, with the rest due to be added by the end of next year.

Moreover, Riley said, even when the system detects people who have overstayed their visa status, there may not be enough enforcement officers to track them down.

Some experts are skeptical of the government's ability to effectively bring together all of the various stores of information from dozens of law enforcement agencies. In an ideal world, local police officers, airport officials and U.S. Customs officials could easily consult an integrated database of information to check on a potential suspect.

But such systems often are prone to errors.

"There's always some data that is just wrong," said David F. Kotz, a Dartmouth College computer science professor who specializes in security. Moreover, the government has a mixed track record in bringing together disparate databases.

In fact, DHS still has not completed a congressional mandate to merge various agency watch lists of suspects. Homeland Security Secretary Tom Ridge has pledged completion of the project by the end of the year.

Privacy advocates also fear that RFID technology would be extended to monitor the movements of visitors after they enter the country. Such a system would require a series of electronic scanners capable of detecting and reading an RFID signal at long distances and is not currently contemplated, according to the government.

A 2002 General Accounting Office report also questioned whether RFID technology for homeland security was the most cost-effective technology for border security. Supporters said that the technology has been refined since then and that the price of the chips used in readers has dropped.

Staff writer Anitha Reddy contributed to this report.