Purdue University students are testing new software that may track and warn about tornadoes, such as this one which struck Rochelle, Ill., in early April. Walker Ashley/AP hide caption

itoggle caption Walker Ashley/AP

A police officer walks out of the Electrical Engineering Building on the campus of Purdue University in West Lafayette, Ind., Tuesday. One person was killed in a classroom by a gunman who surrendered to a police officer within minutes of the attack, officials said. Michael Conroy/AP hide caption

itoggle caption Michael Conroy/AP