Dec. 11, 2009 -- For millions of travelers who visit foreign countries, it's not just their passports, but their fingerprints, that they need to hand over at the border.

Because fingerprints are unique to each individual, fingerprint scanning at airports and other ports of entry helps security officials quickly identify criminals, terrorists, immigration violators and others. While not foolproof, for some countries, the system is the first line of defense against enemies of the state and other unwanted visitors.

But, earlier this month, Japanese officials arrested a Chinese woman who took a particularly extreme measure to evade detection. She paid a plastic surgeon to surgically alter her fingerprints.

Lin Rong, 27, was originally arrested Dec. 5 for faking a marriage license after she'd already entered the country, Japanese officials said. That' s when police saw scars on her hands and realized that she might have been guilty of more.

Surgeon Swapped Fingerprints From Woman's Right and Left Hands

"During the course of the investigation, police found that she passed through the checkpoint using fake fingerprints," said Nobuyuki Kawai, director of the International Crime Office of Japan's National Police Agency.

He said Rong had previously been deported but sneaked back into the country to work in a restaurant or a bar.

Although local reports said she paid about $15,000 for the fingerprint transplant surgery, Kawai said Rong told officers she paid roughly $1,500 for the procedure that swapped fingerprints from her right and left hands.

Patches of skin from her thumbs and index fingers were reportedly removed and then grafted on to the ends of fingers on the opposite hand. As a result, Rong's identity was not detected when she re-entered Japan illegally.

Japanese Official: Immigration Officers Tightening Security

Since Japan started checking the fingerprints of foreign visitors in 2007, Kawai said, immigration officials have caught eight other people who tried to alter their fingerprints through various techniques.

But, he said, this was the first time officials had identified an illegal immigrant who used surgery to fake fingerprints.

"This is the first case," Kawai said. "But please keep in mind that we don't know if there are any other cases. You never know unless you catch the person."

Rong, who is still in custody, could either serve time in a prison or be immediately deported, he said. And Kawai added that the immigration department is tightening security at airport checkpoints.

"We are sure they are trying to be more careful at border control," he said. "So we hope we can prevent similar cases in the future."

Could Fingerprint Fraudsters Sneak Into the U.S.?

But should this example raise a warning flag for the United States and other countries that use fingerprint scanning and other biometric security systems?

Although Rong's method of deception was especially drastic, she was not the first to figure out how to fool a fingerprint scanner.

In January of this year, it was reported that a South Korean woman blocked from entering Japan apparently slipped past the screening system by placing special tape over her fingerprints.

The silicon covering foiled the scanning device and didn't alert officials that she had been deported in 2007 for overstaying and was barred from re-entry for five years. Immigration officials later found her inside the country.

Bruce Schneier, a security technologist and author, said he was familiar with techniques that could defeat fingerprint scanners in the lab, but was surprised to hear that someone had actually succeeded at the airport.

"I'm not surprised that it's possible but that someone did it," he said. "The real-time, high-stress environment of an immigration line is a little different."

Security Expert: Criminals Must Defeat Technology and Social System

Criminals have tried for decades to fool fingerprint scanners, he said, but what works in an isolated context doesn't necessarily work in the real world.

"It's not just defeating the technology," he said. "You need to defeat the social system around it."

Although officials with the Department of Homeland Security declined to speak on the record, they told ABCNews.com that immigration officers at airport checkpoints are trained to detect and thwart techniques biometric fraudsters might use.

They also said that as opposed to Japan's fingerprint scanner, which only scans two fingers, scanners at U.S. airports scan up to 10 fingers of foreign visitors.

"What happens because of the way our system works, the officers are right there when the fingers are scanned," one DHS official said. "If any question comes up, they can look at the person's hand."

At any one time, about 5 million people are on the country's biometric watch list, for crimes, drug offenses, immigration violations and other reasons, a DHS official said.

Homeland Security: Officers Trained to Detect and Catch Fraud at the Border

Although they couldn't disclose a precise number, the officials said that since the U.S. implemented its fingerprint scanning system in 2004, several people had attempted to evade detection through various kinds of fingerprint alteration and mutilation.

Some attempted deliberate scarring, others used acid to burn their fingers.

A few years ago, they said, someone attempted to graft skin from their toes onto their fingers. But because the wounds had not yet healed, the officer noticed the cuts and denied the person entry.

The officials said they had not seen an increase in the number of fingerprint modification cases and emphasized that Rong's success would not have been possible here.

"We're on the lookout for it and officers are trained to look for this kind of alteration," one official said. "We would be able to detect this kind of thing and prevent it from happening."