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.