Tennessee's driving certificate for illegal immigrants isn't valid as a form of ID, but people are paying hundreds of dollars on the black market and traveling hundreds of miles to get one.

Tennessee has issued more than 51,000 certificates since it became the first state to offer them in July 2004, but not every certificate has gone to someone living there.

Two major federal arrests in recent months exposed shuttles bringing South and Central American immigrants from as far away as New Jersey to state licensing centers in Knoxville, where the immigrants got certificates using fake residency papers.

Last week, a third sweep revealed an alleged conspiracy in which prosecutors say state license examiners in Murfreesboro, outside Nashville, accepted bribes to provide illegal immigrants with driver's licenses and certificates without testing.

"We have seen individuals coming to Tennessee to take advantage of the driver's certificate program because they are easy to obtain," said acting U.S. Attorney Russ Dedrick.

The disclosures come as Tennessee's certificate system is being studied as a possible model for handling "non-conforming drivers" under the Real ID program recently enacted by Congress that will set a national standard for driver's licenses by 2008.

Although the words "not valid for identification" appear in bold red letters on the face of the wallet-size certificates, Dedrick said banks accept them as legal ID and they "can easily be passed off for other types of identification documents."

Lawyer Mike Whalen, who represents a woman accused of bringing as many as 100 immigrants from New Jersey to Knoxville for certificates, said the government is making too much of the problem. His client represented workers, not terrorists, he said.

"Somebody went through the roof and said, 'Remember 9-11, every one had driver's licenses,'" he said. "Well, none of these Mexican immigrants are in flight school anywhere. There is a difference."

That argument carries little weight in law enforcement circles.

The certificate law "just kind of opened up a flood gate of everyone wanting to come here to get some sort of identification," said Knox County Sheriff Tim Hutchison, whose officers discovered that 58 illegal immigrants used the same Knoxville address to get certificates.

Applicants must provide two documents, such as utility bills or a lease, to show they live in Tennessee, and a Social Security number — or a sworn affidavit if there is none. They also must pass an eye exam, a driving rules test and a road test.

"What we tried to do in Tennessee was to recognize that there are people who may be legally here but they are not completely documented," Gov. Phil Bredesen said.

Tennessee had started licensing illegal immigrants, without a Social Security number requirement, in early 2001. More than 180,000 obtained licenses before 9-11 fears set in. The driving certificates were created in 2004 to satisfy homeland security concerns while allowing illegal immigrants to drive with certified proficiency.

Some say the problem isn't the law, it's the enforcement.

Joan Friedland, an immigration policy attorney with the National Immigration Law Center in Washington, said the key is "rigorous proof of state residence."

Hutchison's officers searched the Internet to determine that immigrants were using fake residences, and then spent months monitoring the suspects' movements before they were arrested.

"I would hope that the state would pick up on it sooner," the sheriff said. "But I am not sure that they are actually geared to do that."

The governor said the system has been tightened up since he saw an ad in a Spanish-language newspaper in Georgia promoting package deals for "a certain amount of money to get on a bus and go to Tennessee to get a driver's license."

Still, the allure of payoffs to underpaid license examiners may only increase as requirements tighten and certificates become more precious, said Melissa Savage, a policy analyst with the National Conference of State Legislatures.