By Chris Boyette and Darran Simon, 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.

Sallet declined to say how far the suspect got into the building.

The suspect, who is custody, is not believed to be a veteran, Lori Lohar, the medical center's acting director, said.

Chicago PD and VA Hospital police searched the hospital, Sallet said. There is no other known threat at this time, authorities said.

"I cannot tell you with all of the things that are going on in this country right now, how lucky we are and how blessed the city of Chicago is that we are out here talking about a subject who has in custody but did not hurt anyone," Sallet said.

The incident is under investigation, Chicago Police Deputy Chief Christopher Kennedy said.

Authorities declined to discuss a motive or say who the alleged gunman was shooting at. His identity was not released,

Sallet couldn't immediately say if the suspect had any connection to the hospital at this early stage in the investigation.

The US Attorney is evaluating charges because the VA hospital is federal facility, Sallet said.