An FBI supervisor, sounding a prophetic pre-Sept. 11 alarm, warned FBI headquarters that student pilot Zacarias Moussaoui was so dangerous he might "take control of a plane and fly it into the World Trade Center," a congressional investigator said in a report Tuesday.

The supervisor said he had no reason to believe such an attack was planned but made the argument Aug. 27, 2001 -- 15 days before the attacks -- to convince higher-ups of the need for a search warrant for Moussaoui's computer. Moussaoui has since been charged with conspiring in the Sept. 11 attacks.

His effort failed.

The Moussaoui case was one of two glaring examples of FBI agents recognizing the dangers of terrorists striking from the skies in the weeks before the attacks, only to be stifled by legal restrictions, limited resources and bureaucracies, the report said.

In the other example, a Phoenix FBI agent's warning in July 2001 that Usama bin Laden might be sending terrorists to train at U.S. flight schools was deemed "speculative and not particularly significant," said the report by Eleanor Hill, staff director for the House and Senate intelligence committees' joint inquiry into the attacks.

New York-based agents already knew Middle Eastern flight students associated with bin Laden were training in the United States, but believed he wanted them to transport goods and people in Afghanistan, she said.

In presenting her report to the committees, Hill did not suggest that better handling of the Moussaoui case or the so-called Phoenix memo or any other lead would have prevented the Sept 11 attacks. But she said the country could have been better prepared if those cases had been tied together and with other clues.

Among them were topics she discussed last week: the lackluster search before the attack for two men who turned out to be hijackers, and a rise in reports suggesting an imminent terrorist action.

"Clearly, it might have drawn greater attention to the possibility of a terrorist attack in the United States, generated a heightened state of alert regarding such attacks and prompted more aggressive investigation and intelligence gathering," she said.

An FBI headquarters counterterrorism supervisor said the Moussaoui and Phoenix matters weren't connected because of a massive volume of work facing FBI staff, a lack of analytical resources and poor technology.

"It is just extremely difficult for individuals to keep these matters connected and to see everything and to make these connections in their head," he said.

He and two other witnesses spoke behind a screen and their names were not revealed. But one witness was Kenneth Williams, writer of the Phoenix memo, who complained that his identity had been revealed to the public, putting him potentially in danger.

Moussaoui was arrested by FBI agents in Minnesota on immigration charges in August 2001 after a flight school instructor became suspicious of his desire to learn to fly a commercial jet. FBI headquarters denied agents' requests to seek a warrant to search his computer. Hill said FBI agents misunderstood the requirements for getting the special warrant needed for foreign terrorist suspects.

When an anti-terror unit agent told him he was getting people "spun up" over Moussaoui, a Minneapolis supervisor said "he was trying to make sure that Moussaoui `did not take control of a plane and fly it into the World Trade Center,"' Hill said, citing the supervisor's notes and a statement he gave. The supervisor said he made the comments merely to get headquarters' attention.

"That's not going to happen...," Hill said the headquarters agent replied. "You don't have enough to show he is a terrorist. You have a guy interested in this type of aircraft -- that's it." The agent doesn't remember the exchange, Hill said.

The Phoenix memo was prompted by Williams' concerns about 10 foreign-born Sunni Muslims studying aviation issues. One, a member of the militant group al-Muhajiroun, was taking aviation-related security courses at Embry Riddle University.

The memo received little response in part because of racial profiling concerns. Also, it's unclear how many people saw it.

"The FBI's electronic system is not designed to ensure that all addresses on a communication actually receive it," Hill said.

None of the students Williams was investigating were involved in the Sept. 11 hijackings. But one knew hijacker Hani Hanjour from flight training and an Arizona religious center.

In related developments Tuesday:

-- The Senate voted to create an independent commission with broad powers to investigate the events leading to the attacks. The House has approved a more narrow investigation.

-- Federal prosecutors disclosed a link between Moussaoui and hijacker Ziah Jarrah, who was on United Flight 93. A telephone number Moussaoui called was scrawled on a business card belonging to Jarrah, found at the plane's Shanksville, Pa., crash site.

-- Intelligence committee leaders again urged the Bush administration to let them declassify documents revealing White House knowledge about terrorist threats. The White House has refused.