Washington ? Five months before Sept. 11, the government warned airlines that Middle Eastern terrorists could try to hijack or blow up a U.S. plane and that carriers should “demonstrate a high degree of alertness.”

The warning, obtained Saturday by The Associated Press, came out after the April 6, 2001, conviction of Ahmed Ressam in connection with a failed plot to blow up Los Angeles International Airport during the millennium celebrations.

The memo from the Federal Aviation Administration, dated April 18, 2001, also noted that four al-Qaida members were on trial in New York, accused of being involved in the U.S. embassy bombings in Kenya and Tanzania in 1998.

The warning, which expired July 31, was one of 15 information circulars sent last year that warned of potential terrorist threats before Sept. 11.

Bush administration officials have said the threats were so vague that they did not require tighter security. Nor did they envision a scenario similar to Sept. 11, when suicide terrorists turned four commercial airliners into missiles.

Also Saturday, Republicans defended President Bush against criticism that his administration ignored warning signs about the Sept. 11 attacks.

“Americans know that President Bush, when faced with credible information about a threat, would act swiftly and strongly,” Republican Party Chairman Marc Racicot wrote in an e-mail to GOP supporters.

Any statements that suggest “anything to the contrary” are “irresponsible and politically motivated,” Racicot wrote. Democrats are suggesting an expansion of inquiries into what the White House and federal law enforcement knew about possible terror attacks.

Sen. Richard Shelby, vice chairman of the Senate Intelligence Committee, charged that the FBI “was either asleep or inept, or both,” for failing to act on a July 2001 memo from the agency’s Phoenix office about Arabs seeking U.S. flight training. Democratic criticism of the president is “a bogus charge” and “he didn’t know more than we basically knew,” Shelby, R-Ala., told CNN.

But a committee Democrat, Sen. Ron Wyden of Oregon, said he thought it was a misuse of the trust that Congress and the public put in the administration after Sept. 11 “to call anybody irresponsible who asks tough questions about what sure looks like an intelligence failure.”

From New York, Democratic Sen. Hillary Rodham Clinton said of the questioning: “The point is not to point fingers or place blame but to be sure we have learned and we are better prepared.”

The airline memos obtained by the AP do not provide specific details about the threats, nor do they instruct the airlines to follow new security procedures.