Assuming the answer you're looking for is "Canada", do you have a link?
It's my understanding that none of the 9/11 attackers entered from Canada.
Two of the hijackers apparently came to the United States from Nova Scotia, Canada, crossing the border via a ferry to Bar Harbor, Maine. The Royal Canadian Mounted Police and Interpol are assisting U.S. law enforcement in retracing these hijackers' steps in Canada.That article is from 2001, and facts since then may support your contention that none of the 9/11 hijackers entered the US via Canada. But in my opinion, the US/Canada border is so porous and heavily traveled that it makes sense that at least some of the hijackers entered the US through this border.