BLACKSBURG, Va.—Haiyan Cheng remembers the black gun. In the split second the Virginia Tech doctoral student glimpsed Cho Seung-Hui coming out of a classroom in Norris Hall on April 16, one thing stood out: the black-clad young man's long, black gun, clutched in his right hand.

Cheng wasn't supposed to be in Norris Hall that day. The 36-year-old research assistant rarely teaches, but agreed to fill in for an out-of-town professor. On her way to class, she saw Liviu Librescu, a 76-year-old engineering professor, rummaging through his briefcase in the classroom across the hall. Less than an hour later, Librescu, a Romanian-born Holocaust survivor who taught for 20 years at this 135-year-old university in the rolling green hills of the Blue Ridge, was dead. And Cheng was praying for her life.