At least five of the Sept. 11 hijackers exploited a glaring hole in visa security to stay in the U.S. beyond their time — allowing them to board the planes and conduct their murderous attack. Fifteen years later, and despite a clear consensus on the need for improvement, that gaping hole remains. “In too many cases that’s still happening — they come in legally, but we don’t know if they’ve left, and if they haven’t left, we don’t know where they are,” said Tom Kean, former governor of New Jersey and chairman of the 9/11 Commission that looked into the...