More than 33 people were killed Monday on the campus of Virginia Tech in Blacksburg, Va., after a gunman opened fire in a dormitory and a classroom building. More than 20 others were injured.

The gunman, who also died, has not yet been identified.

Virginia Tech, a large university in the southwestern part of the state, has more than 25,000 students on a 2,600-acre campus. Gunfire was first reported in a dormitory at 7:15 a.m. ET. The shooter then moved across campus to an engineering building where the second round of shooting occured.

The attack was the deadliest shooting ever on an American campus. Virginia Tech president Charles Steger described it as a tragedy of "monumental proportions" when he spoke with reporters.

Scott Hendricks, associate professor of engineering science and mechanics at Virginia Tech, tells Michele Norris that he was on the third floor of Norris Hall when the second round of shooting happened. The building was the site of most of the deaths.

Hendricks says he barricaded himself in his office, and that he heard 30 to 40 rounds fired over a period of about 30 minutes. Two of his colleagues were shot — one in the arm, the other in the face. Hendricks says he and some students hid in a classroom until it became quiet enough to leave.

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