Mohamed Atta was a busy man prior to the Sept. 11 terror attacks, checking out flight schools in Oklahoma and Florida, meeting Islamic extremists in Spain, inquiring about crop dusters in Florida, conferring with an Iraqi intelligence agent, skipping a traffic court date. 

Atta was so busy, in fact, that the trail he left behind has made him a key figure in efforts by investigators to link the multiple attacks and trace their source.

Sen. Charles Grassley, R-Iowa, said public reports of Atta's activities before the attacks suggest that he may well be ``the pied piper of the hijackers'' who nearly simultaneously crashed four jetliners in New York, Washington and Pennsylvania, killing thousands of people.

``It makes sense that investigators are exploring Atta's role so thoroughly,'' said Grassley. ``Atta's background may yield critical insight into the terrorists' organizations and mind set.''

Atta, 33, believed by authorities to have been at the controls of the plane that slammed into the World Trade Center's north tower, has been connected publicly with suspected hijackers on two of the other three flights involved in the attacks.

Atta, Marwan Al-Shehhi and Ziad Jarrah studied together in Hamburg, Germany, during the 1990s, and German authorities say they believe the three were part of a cell formed there early this year to attack targets in the United States. Two other alleged members of the cell are being sought by German authorities on warrants charging each with more than 5,000 counts of murder.

U.S. authorities have said Al-Shehhi, believed to be a cousin or nephew of Atta, piloted the plane that hit the trade center's south tower. Jarrah was believed to have been on the hijacked United Airlines flight that crashed in a field in southwestern Pennsylvania.

Witnesses also have placed Atta in a suburban Washington motel room where five men stayed from late August through Sept. 10. The motel is less than a mile from another motel where Jarrah and Nawaq Alhamzi were registered on separate nights in late August and early September. Alhamzi was on the plane that crashed into the Pentagon.

Unlike some suspected hijackers who used stolen identities to confuse authorities, Atta mostly used his own name and vital statistics as he traveled the country and the world in the months before the hijacking.

When Broward County authorities stopped Atta on April 26 for driving without a license, he gave them the same name and date of birth the FBI released for him after the attacks.

Atta failed to appear in court on the traffic charge, and an arrest warrant was issued on June 4. But officials said the warrant would not have surfaced unless Atta was involved in another traffic stop or criminal activity.

``There's over 200,000 warrants in the system,'' said Veda Coleman-Wright, a spokeswoman for the Broward County sheriff. ``So naturally you're going to make sure you're going out and getting those wanted for murder.''

The same month the warrant was issued, Atta traveled to Las Vegas and spent two nights in a motel just around the corner from the FBI office. The hotel's registration receipts showed he had a Florida driver's license, a 1968 birth date and an address in Coral Springs, Fla.

In July, Atta used a bad credit card at the Government Printing Office in Washington to try to purchase the official Air Force magazine. GPO spokesman Andrew Sherman said Atta gave an address in Egypt.

The same month, according to Spanish newspapers, Atta met with several Islamic extremists in a hotel at a beach resort near Barcelona.

U.S. officials disclosed after the attacks that information from a foreign intelligence service indicated that Atta met earlier this year in Europe with an Iraqi intelligence agent.

Both Atta and Al-Shehhi received flight training in Venice, Fla., Tampa, Fla., and Miami after visiting a flight school in Norman, Okla. The same Norman school provided entry flight training to Zacarias Moussaoui, who has been detained in Minnesota since mid-August after he aroused suspicions by seeking training on a simulator to fly — but not land — passenger jets.

In the south Florida town of Belle Glade, a group of Middle Eastern men, including Atta, repeatedly visited a fertilizer company asking about crop duster planes in the weeks leading up to the attacks.

Attorney General John Ashcroft told the House Judiciary Committee on Monday that the FBI had gathered information raising fears that the small farm planes could be used in a biological or chemical attack. He said Atta had shown interest in crop-dusters and another person now in federal custody had downloaded information about the planes from Internet.

``There is no clear indication of the time or place of these attacks,'' Ashcroft said.