Saudis
It's common knowledge that the Saudis have funded mosques around the world, including here, and that these mosques are primarily "wahhabi," a more fundamentalist form of Islam. This is not a good thing, but still a far cry from saying they were responsible for 9/11.
Al Qaeda has people from many nations in it. They selected mostly Saudis for their 9/11 operation for the obvious reason that it was much easier to get visas for them and have them move around the U.S. without raising suspicion. This is specifically mentioned in the quote.
In more recent times, they've worked on recruiting Westerners and citizens of target countries for the same reason.