The federal government's "no-fly" list had 16 names on it on Sept. 11, 2001. Today, it has more than 20,000.

The list, which identifies suspected terrorists seeking to board commercial airplanes, expanded rapidly even though the government knew that travelers were being mistakenly flagged, according to federal records. The records detail how government officials expressed little interest in tracking or resolving cases in which passenger names were confused with the growing number of names on the list.

More than 2,000 people have complained to the Transportation Security Administration. Airlines, at one point, were calling the agency at least 30 times a day to say that they had stopped a passenger whose name was similar to one on the list but after further investigation was determined not to be a terror suspect, according to a TSA memo.

More than 300 pages of documents related to the no-fly and related lists were released late Thursday night by the TSA and the FBI in response to a federal court order. The American Civil Liberties Union had filed suit on behalf of Jan Adams and Rebecca Gordon, two peace activists who wanted to know why their names had turned up on a no-fly list.

The documents reveal early symptoms of what are now known to be flaws with the watch lists. Travelers who were flagged by the lists said they now foil the system by altering how their names are spelled on their tickets -- adding their middle initials, full middle names or titles, for example.

Government officials do not announce when they stop passengers actually on the lists. The only publicly known case involved Yusuf Islam, once known as the pop singer Cat Stevens, who was prevented last month from entering the country.

The information revealed by the documents is "not very comforting," said Thomas R. Burke, a San Francisco attorney representing the peace activists and the ACLU.

The TSA acknowledges that the system for checking passenger names for suspected terrorists needs fixing, and it plans to overhaul it in a new program called Secure Flight. The Justice Department declined to comment.

The false matches "underscore the need we have to get more information on passengers to adjudicate those that are not a risk," said Department of Homeland Security spokesman Brian Roehrkasse.

Every time a passenger books a ticket, the airline checks the traveler's name against two enormous government databases, or watch lists, of people the government believes pose a threat. The FAA created two lists in 2001: a no-fly list and a so-called selectee list, both of which airlines compare against reservation records. When the TSA was formed in 2002, it took over maintenance of the lists from the FAA. The no-fly list grew from 16 names supplied by the FBI in 2001 to 1,000 names by the end of 2002, according to the newly released TSA documents. There are now more than 20,000 names on the no-fly list, some which are aliases, according to a homeland security source who is not allowed to release such numbers. There are several thousand names on the selectee list, according to the source.

Internal TSA memos direct airlines to refuse boarding to a passenger on the no-fly list and to alert the local FBI. Travelers on the selectee list are to be directed to a law enforcement officer and put through additional security procedures in order to board the plane, the documents said.

Airlines declined to say exactly what kind of technology they use to match names. But the documents make clear that in the months after Sept. 11, carriers were having difficulty with the task. The Air Transport Association, the airline trade group, met with the TSA's top policy director in December 2002 to address the "false positives problem," according to a TSA memo.

"This has been such a headache for me," wrote one Alaska Airlines executive, whose name was redacted, in an e-mail to the TSA a week before the meeting. "Any solutions . . . would be greatly appreciated."

TSA officials wrote letters and e-mails of apology to passengers who complained of being mistakenly flagged by the lists. But in an internal memo, officials said there was little the agency could do.

"While a few carriers keep track of 'false positives' the majority do not," wrote Chad Wolf, now TSA's number-two policy official, in a December 2002 e-mail to agency legislative affairs official Cori Sieger. "Consequently, TSA does not have the ability to record this data nor is there a pressing need to do so."

Passengers are falsely flagged by the lists in such large numbers because of the kind of technology airlines use to compare the reservation lists to the watch lists, according to experts in name-matching technology. Each airline conducts the matches differently. Many major carriers use a system that strips the vowels from each passenger's name and assigns it a code based on the name's phonetic sound, according to the Air Transport Association.

The name-matching technology is "too simplistic for a very complex problem," said Jack Hermansen, co-founder of Language Analysis Systems Inc. in Herndon, a company that has a competing name-matching technology that factors in a name's cultural origin. "It's these accidental matches that cause the big problem."

The phonetic-code concept is traced back to a program called Soundex patented in 1918, which was used by Census Bureau officials to help sort out names that sounded similar but might be spelled differently. The name "Kennedy," for example, would be assigned the Soundex code K530, which is the same code assigned to Kemmet, Kenndey, Kent, Kimmet, Kimmett, Kindt and Knott, according to genealogy Web sites that use the technology. Today's systems are more sophisticated than Soundex, but they grew from the same origins, experts said.

"The reason this technology is used is you're really trying to protect against typing errors," said Steven Pollock, executive vice president at TuVox Inc., a company that sells speech-recognition software. "When someone types in a name, the problem and the challenge is people will spell names incorrectly. . . . Names are definitely the toughest things to get [right], no doubt about it."

But the phonetic coding systems tend to ensnare people who have similar-sounding names, even though a human being could tell the difference. Earlier this month, for example, Rep. Donald E. Young (R-Alaska), said he was flagged on the "watch list" when the airline computer system mistook him for a man on the list named Donald Lee Young.

ACLU staff attorney Jayashri Srikantiah decries no-fly lists. Clients Jan Adams, right, and Rebecca Gordon sued the government after being detained.