Armed guards, metal detectors and X-ray machines greet visitors to the U.S. Holocaust Memorial Museum. Traffic barriers keep away trucks like the one that blew up the Alfred P. Murrah Federal Building in Oklahoma City.

Since long before the museum opened in 1993, officials have been preparing for the possibility of a hate-fueled attack. But until an elderly white supremacist allegedly walked in with a rifle and shot a security guard Wednesday, there had never been one.