The Phoenix-based agent, Kenneth Williams, could not be reached for comment Friday about the July 2001 warning.
But his former colleagues at the FBI said Friday that Williams' knowledge of terrorism alone should have been enough for superiors to immediately act on his suspicions.
"Nobody listened to him," said one top former FBI official who first learned about the memo several weeks before its existence surfaced publicly, creating a firestorm in Congress.
Other former colleagues of Williams, who refused to discuss the memo, offered high praise for the agent's work in the Phoenix counterterrorism unit.
"Anyone in FBI management who wouldn't take what Ken Williams said seriously is a fool," said Ronald Myers, who served for 31 years in the FBI before retiring in 2000. "If Ken says something, it's true."
Added Williams' former FBI swat team leader Roger Browning: "On a scale of 1 to 10, he's a 10. Maybe an 11."
The now-famous memo warned that possible terrorists were seeking access to flight training schools and other U.S. aviation facilities.
"Phoenix believes that the FBI should accumulate a listing of civil aviation universities/colleges around the country," the memo says in part, urging officials in Washington to relay the bureau's "suspicions" of terrorist infiltration with "other elements of the U.S. intelligence community."
The Bush administration has stressed that none of those identified in the memo were linked to the Sept. 11 attacks on the World Trade Center and the Pentagon.
The memo, which reportedly was sent to FBI terrorism experts for further analysis, did not reach senior levels of the bureau before Sept. 11, FBI officials said.
Williams, 41, a former San Diego police officer who for a time was assigned to a border crimes task force, joined the FBI's Phoenix office a little more than a decade ago.
At one point, he was a SWAT team leader who--during one search--braved pneumonia while leading agents in a successful hunt in mountainous terrain for a father and son wanted for killing a police officer.
But it was in counterterrorism that Williams really made his mark, several former colleagues said Friday.
"He is one of the sharpest agents I ever met," said Myers, who is now a court constable in Maricopa County, Ariz.
"Of all the people I knew and worked with in the Phoenix office, I'd put him in the top three or four."
Added a former partner of Williams: "He is simply one of the best agents in the FBI and one of the best international terrorism agents in the world."
One official in Washington described Williams as "pretty strait-laced and not a whistle-blower type."
The spokesman for the Phoenix FBI office, Special Agent Manuel Johnson, would say only that Williams "is an experienced agent" who "continues to be assigned to the Phoenix division."
Although Williams' memo has only recently drawn public attention, records and interviews show that as early as 1998, FBI agents in Phoenix were attempting to tap the city's Muslim community for information about potential terrorism.
Since the attacks, Williams has played key roles in the FBI's Arizona investigation of associates of the Sept. 11 hijackers, particularly Hani Hanjour, the young Saudi suspected of piloting the commercial airliner that struck the Pentagon.
Williams testified in the trial earlier this year of another Saudi living in Arizona, Faisal Al Salmi, who was convicted of lying about having known Hanjour.
Williams also was among the agents who interrogated Malek Seif, a Middle Eastern man who was an acquaintance of Hanjour's and left the U.S. shortly before the Sept. 11 attacks, according to Seif's attorney, Thomas Hoidal.
Seif was later lured back to Phoenix as part of the terrorism investigation and charged with making false statements to obtain a Social Security card and lying on an immigration asylum application.
Seif recently pleaded guilty, was sentenced to time served and has returned to France.
Earlier inquiries by the Phoenix FBI bureau into the activities of flight school students appear to reflect the concerns expressed in the summer 2001 warning to FBI headquarters.
More than a year before the memo was written, FBI agents in Phoenix were monitoring Mideast-born residents, including a Lebanese student at Embry-Riddle Aeronautical University in Prescott, Ariz.
Zakaria Soubra, who endorsed a radical Islamic doctrine of Muslim world rule, told The Times that agents showed up at his apartment in early 2000 to question him.
The agents were concerned, Soubra said, about his visit to a shooting range with another Muslim, who was a veteran of Islamic jihads in the Balkans and the Middle East.
And within hours of the Sept. 11 attacks, agents returned to Prescott to question Soubra and several fellow Muslim students at Embry-Riddle.
Soubra was not charged with any crimes and is now a senior at the campus.
Times staff writer Richard Serrano and Times research librarian Nona Yates contributed to this report.