WASHINGTON -- Airport and airline security was not increased before Sept. 11 despite the government's fears that Osama bin Laden's terrorist group might be planning hijackings. Federal regulators said they warned the airlines, but the information was too vague to force changes.

Pilots' and flight attendants' unions say their members were never told. Airlines and other industry officials said any warnings they received were not about a specific threat.

In a written statement Thursday, American Airlines said it "received no specific information from the U.S. government advising the carrier of potential terrorist hijacking in the United States in the months prior to Sept. 11."

American said it receives Federal Aviation Administration security information bulletins periodically, but they were "extremely extremely general in nature and did not identify a specific threat or recommend any specific security enhancements."

Other airline officials, who until Feb. 17 were responsible for airport security checkpoints, and the companies that screen passengers, said any warnings they got did not contain a specific threat that would prompt action.

"Carriers receive security alerts and cautions from time to time in their daily interface with government agencies, and are typically very general in nature," said Joe Hopkins, a spokesman for United Airlines, which lost two planes on Sept. 11.

"During 2001, there were no alerts or cautions that indicated a Sept. 11th scenario was credible or possible."

Boston's Logan Airport, where two of the hijacked planes took off from, "was never provided with intelligence information that indicated an increased risk of an aircraft hijacking from Logan," said Jose Juves, spokesman for the Massachusetts Port Authority.

Jeff Zack, a spokesman for the Association of Flight Attendants, said the group's members were not told anything before Sept. 11. "If we're supposed to be looking for suspicious people, we should know we're supposed to be looking for them," Zack said.

At the end of July, the FAA warned airlines and airports that terrorists might be planning hijackings, said Condoleezza Rice, the president's national security adviser.

She said they were told that "there's no specific target, no credible info of attack to U.S. civil aviation interests, but terror groups are known to be planning and training for hijackings, and we ask you therefore to use caution."

The warning followed other, more general warnings to airlines in June and July that cautioned of the possibility of an attack, especially abroad, Rice said.

At least one FAA alert warned that terrorists might hijack a plane so they could trade the passengers for Sheik Omar Abdel-Rahman, imprisoned for plotting to blow up New York landmarks, officials said.

"All of this reporting about hijacking was about traditional hijacking," Rice said. "They were mostly worried that they might try to take a plane and use it for release of the blind sheik or some of their own people."

One warning named Osama bin Laden and said his or other terrorist groups could hijack airplanes, a government official said Thursday.

The FAA declined to release copies of the classified warnings, which are sent to airline security directors or posted on a secure Web site.

The FAA did not order new security measures in response to the warnings, agency spokesman Scott Brenner said.

There was no information about when an attack might occur or the possibility of terrorists crashing hijacked airplanes into buildings, officials said.

FAA Administrator Jane Garvey and other agency officials receive regular intelligence briefings and for years have been aware of threats made by bin Laden, Brenner said.

"This is a threat we've been watching intensely since 1998 when bin Laden made some very public statements," Brenner said. "While we were watching these groups, we never had a credible hijacking threat. It was never, 'This group was going to do a hijacking.'"

Airline experts said the FAA should have tightened security after receiving the warnings.

"With that threat escalating, why was it permissible to continue to take cutting tools on airplanes, and why did we have a flawed computer assisted passenger profiling system that didn't require a search of the passenger and carryon articles?" former FAA security chief Billie Vincent said.