(CNN) A gunman armed with an assault rifle opened fire Monday on the campus of Jesse Brown VA Medical Center in Chicago, the FBI said.

The gunman initially began firing at the building, said Jeffrey S. Sallet, the FBI special agent in charge of the Chicago office, then entered the building firing.

Within 30 seconds, VA police responded and stopped the alleged gunman without firing a shot, Sallet said. He didn't say how officers stopped the gunman.

No was hit by gunfire or injured in any way, Sallet said.

"We avoided tragedy here in the city of Chicago today," Sallet said.

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