Fifteen years after the September 11 terrorist attacks, the United States government still has little idea how many foreign travelers overstay their visas annually and remain in the U.S. This, despite the fact that two September 11 hijackers, Satam al-Suqami and Nawaf al-Hazmi, overstayed their visas before carrying out the deadly attacks that left nearly 3,000 Americans dead. Two other 9/11 hijackers had also previously overstayed their visas.