Did Americans aid terrorists?

Might be a good idea to start with the FBI agent who roomed with two of the hijackers.

Did Americans aid terrorists? - Hardball with Chris Matthews - MSNBC.com:

Prior to the attacks on the World Trade Center and the Pentagon, all 19 of the terrorists had been living in the U.S. for several months, some for more than a year. They obtained visas, signed apartment leases, shopped, prayed at mosques, rented cars and bought airline tickets.
Could they have done so, especially those who did not speak English, without help from American citizens? It is one of the top unanswered questions lingering about the 9/11 attacks.