On Sept. 11, 2001, 19 people hijacked four U.S. commercial airliners after those airliners took off from airports inside the United States. They then flew those airliners into the World Trade Center towers, the Pentagon and a field outside Shanksville, Pa. The hijackers had certain characteristics in common. All were young males from regions of the world that in recent decades had produced radicalized Muslims -- and the U.S. government had given all of them visas to enter the United States. The oldest of the hijackers was 33; the youngest was 20; their average age was about 24. One came...