What was the Saudi connection to 9/11?

Fifteen of the 19 hijackers on 9/11 were from Saudi Arabia, which was also the home of al-Qaeda’s leader, Osama Bin Laden. In the days after September 11, dozens of his friends, family members and business associates left the U.S. on secret flights bound for The Kingdom of Saudi Arabia. That has led to questions that four presidents have avoided answering:

What role did the Saudi government play in the attacks? What did it know?

Even worse, did it help?