Airlines were warned in a memo from the Federal Aviation Administration that Middle Eastern terrorists could try to hijack or blow up a U.S. plane five months before Sept. 11.
Carriers should "demonstrate a high degree of alertness," according to the warning, set to expire July 31, 2001, and obtained Saturday by The Associated Press.
The memo, dated April 18, 2001, came out after the April 6 conviction of Ahmed Ressam in connection with a failed plot to blow up Los Angeles International Airport during the millennium celebrations.
It also mentioned that four Al Qaeda members were on trial in New York, accused of being involved in the U.S. embassy bombings in Kenya and Tanzania in 1998.
This warning was just 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 stepped up their defense of 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.
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.
The April memo, for example, cited "reports that prompt concern about the safety and security of U.S. citizens traveling through the Middle East." The FAA said the potential for a terrorist attack was high, but there were no credible threats against U.S. airlines.
"Nevertheless, some of the currently active groups are known to plan and train for hijackings and have the capability to construct sophisticated (bombs) concealed inside luggage and consumer products," the memo said. "The FAA encourages all U.S. carriers to demonstrate a high degree of alertness."
On June 22, citing "unconfirmed reports that American interests may be the target of terrorist threat from extremist groups," the FAA again alerted airlines.
The security warning, scheduled to expire Aug. 22, talked about a potential hijacking as a way to secure the release of Sheik Omar Abdel Rahman, imprisoned for plotting to blow up New York landmarks in 1993.
"Although we have no specific information that this threat is directed at civil aviation, the potential for a terrorist operation, such as an airline hijacking to free terrorists incarcerated in the United States, remains a concern," the warning said.
On Aug. 28, in a memo to expire Nov. 30, airlines were warned that fighting between Israelis and Palestinians had led to threats against airlines flying to and from Israel.
The government said it was concerned about the increased ferocity of Palestinian suicide bombings directed against Israeli civilians, as well as an unconfirmed report in the Arab media that foreign airlines were warned to stay away from Israel.
"At this time," the memo said, "the FAA does not have information of a specific threat against U.S. air carriers operating in Israel, but is concerned about the increasing lethality of the bombing attacks and the information regarding targeting of Ben Gurion International Airport."
The Associated Press contributed to this report.