U.S. anti-terrorism rules require airlines and immigration services to check traveler's names against an ever-expanding list of terror suspects and criminals. But how can you match a foreign name when that name can be spelled many different ways in English?

Last November Pakistani-born Mir Aimal Kasi was executed for the 1993 murder of two employees of the Central Intelligence Agency.

He had entered the United States a few years earlier undetected, even though his name was on a terrorist watch list. "He got a visa, crossed the border, purchased an AK-47 and shot five people in front of the CIA. His name was on all the watch lists," said John Hermansen, co-founder of the Virginia-based Language Analysis Systems. "He was a persona non grata in the United States but because of the difference of one letter in his name, he was able to do all those things, get a visa, cross the border, pass the gun check. So that's probably the best-documented and most egregious case of what happens when we don't pay attention to the issue of personal names."

Mr. Hermansen has developed a name reference library and name-search software to help prevent such slips in the future.

Understandably, the demand for name-search technology soared after the 2001 terrorist attack against the World Trade Center.

The key to a thorough search, Mr. Hermansen says, is understanding the culture of the name and the information it contains. "A name can tell you the gender of a person, maybe the religion, social status of that person," he said. "Something about family or marriages or siblings is indicated in some cultures. You can find out what country the name is commonly used in."

Over the past two decades Mr. Hermansen's company has complied a reference library of more than one billion names from just about every country in the world.

But that is not enough. Mr. Hermansen and others working in the field have put together lists of name variables to facilitate the search for a match. "So you'll see the name like Mohamed, always spelled the same way in Arabic," continued John Hermansen. "We have almost 200 variations of it in our data base that have shown up that people would want to match if they found that the other names in the string also matched. So as names are heard and written down, you have names like Abd el Rahman that's really pronounced more fluently so it may occur all as one string. Those two forms of that name would have to be also matched."

Knowing the origin of the name can help a bank or an INS agent detect a fraud. "For instance, in a Russian name at the end you would see a Romanov spelled with a 'v' or 'off' but never with a 'ph'," he said.

The CIA and the FBI are relying more on such extensive name-search systems in their hunt for terrorists. So is the INS, which has to vet immigrants and non-immigrants entering the country. Airlines and banks now are required to check customer names against terrorist watch lists too.

Richard Wagner runs the New York-based Intelligent Search Technology firm, which provides computer technology to speed the data base search. Using my name as an example, he shows just how fast the search can be. "I've just typed the name into a name field and I'm about to press the 'run' button," he said. "When you press the 'run' button essentially what happens is you're asking the system to go out and perform a search. I just performed the search and you'll be delighted to know that you do not exist on any of the terrorist lists."

Mr. Wagner says companies and law enforcement agencies can check about 4,000 names a second, including different spellings or nicknames. "When they type in Bill Clinton, should he be on any of the database lists, they'd wouldn't only find Bill Clinton, they'd find Billy Clintons and William Jefferson Clinton," he said.

Investigators also are using name search systems to help trace the money trail of suspected terrorists.

But the technology has other practical applications too.

Mr. Hermansen of Language Analysis Systems says companies also use the search tools to help identify target markets - like women or minorities. He says the information they need to do that is in the name.