WASHINGTON - Two weeks before the terrorist attacks in New York City and Washington, D.C., FBI agents were at a flight school in Oklahoma asking questions about a man now suspected of having a link to those attacks, sources said.

The agents, sources said, were interested in Zacarias Moussaoui, who was arrested August 17 in Minnesota on an alleged passport violation. Moussaoui was in custody at the time of last week's attacks -- being held as a material witness -- but authorities are investigating whether he and others were part of a broader plot to hijack and crash even more jets.

The disclosure came as at least four people are being held as material witnesses into the U.S. investigation into the September 11 terrorist attacks.

More than 50 people are in government custody, held either as material witnesses to the investigation or on possible immigration violations. They are all being questioned about the attacks, the FBI said, and their level of cooperation has varied. Material witnesses are people who are not necessarily considered suspects but are believed to have valuable information and may pose a flight risk.

Almost 200 other people are being sought for questioning in the case.

In another development, Mohamed Atta, one of the suspected hijackers on board the first plane to slam into the World Trade Center last week, met with an Iraqi intelligence official somewhere in Europe earlier this year, U.S. officials said Tuesday.

U.S. officials cautioned, however, that the meeting does not mean that Iraq had a role in last week's terrorist attacks. Investigators are exploring what the contact between Atta and the intelligence officer may mean.

Jetliner training in Oklahoma

The fact that FBI agents were at the Airman Flight School in Norman, Oklahoma, two weeks before any attacks would seem to contradict the agency's assertion that it was not aware of any connection between aviation schools and suspected terrorists.

"There were no warning signs that I'm aware of that would indicate this type of operation in the country," FBI Director Robert Mueller said Monday.

In Oklahoma, Moussaoui had apparently raised suspicious because he sought training in flying commercial jets despite having a lack of experience.

The possibility that pilots were being trained for terrorist plots was revealed earlier this year during testimony at the trial of four men charged with the 1998 bombings of American embassies in Africa. U.S. prosecutors believe Osama bin Laden -- described as the "prime suspect" in the hijackings -- was behind that plot as well. He, in fact, was indicted for the 1998 bombings.

U.S. investigators also believe that two of the dead World Trade Center hijackers had toured the Oklahoma facility, seeking flight training. Those two hijackers later enrolled in a Florida aviation school.

Meanwhile, the FBI was warned six years ago of a terrorist plot to hijack commercial planes and slam them into the Pentagon, the CIA headquarters and other buildings, Philippine investigators said.

Philippine authorities learned of the plot after a small fire in a Manila apartment, which turned out to be the hideout of Ramzi Yousef, who was later convicted for his role in the 1993 bombing of the World Trade Center. Yousef escaped at the time, but agents caught his right-hand man, Abdul Hakim Murad, who told them about plots to hijack U.S. jets.

In other developments in the investigation:

-- A grand jury in White Plains, a suburb north of New York, will review evidence and issue subpoenas in the attack on the World Trade Center, according to a report by The Associated Press.

-- Law enforcement officials removed two cardboard boxes and five grocery bags of materials from inside a three-story house in Bayonne, New Jersey, late Monday in an investigation related to the attacks last Tuesday that leveled the World Trade Center.

-- Technicians have examined the so-called black boxes from United Airlines Flight 93, which crashed in Pennsylvania during Tuesday's terrorist attack, but the FBI and the boxes' manufacturer aren't saying what information was recovered.

-- The hijackers left behind materials suggesting they had back-up plans for their mission of terrorism, according to government sources. No details about the plans were given.

The material was said to have been confiscated from one or more automobiles left at the airports where the hijackers began their suicide flights. It was not immediately known whether those automobiles included the car at Logan International Airport in Boston registered to Mohammed Atta, who was on the first jetliner that struck the World Trade Center.