'Was there a foreign government behind the 9/11 attacks?' - Was it Saudi Arabia?

A fascinating article at Vanity Fair reporting that:
'in an adaptation from their new book, Anthony Summers and Robbyn Swan examine the connections between Saudi Arabia and the hijackers (15 of whom were Saudi), the Bush White House’s decision to ignore or bury evidence, and the frustration of lead investigators—including 9/11-commission staffers, counterterrorism officials, and senators on both sides of the aisle.

How can people be so suspicious? Apart from as a result of looking at the evidence that is!