CHICAGO (CBS) — A scare was averted Monday afternoon at the Jesse Brown VA Medical Center, when shots were fired into the hospital by a man with an assault rifle – but nobody was injured.

Jeffrey S. Sallet, special agent in charge of the FBI Chicago Field Office, said the offender was carrying an assault rifle. He said VA police mitigated the threat within 30 second of the gunman entering the building.

“We avoided tragedy here in the city of Chicago today,” Sallet said at a media conference Monday night.

As CBS 2’s Charlie De Mar reported, police said the shots started outside and continued inside the VA hospital.

“He walked up and started shooting at the building,” one witness told De Mar. As the shooter stopped to reload, the witness said, “by that time, a car tried to run him over.”

Cellphone video from inside the hospital showed just how quickly hospital police took down the shooter. He was on the ground in less than a minute.

Incredibly, several veterans who spoke with CBS 2 said their first thought when they heard the shot was how they could help and protect others.

“We’re veterans, so we’re thinking not only escape but we’re looking at what can we do,” said witness Dwayne Weems. “So we’re looking for cubby holes; we’re looking for points where we can get a vantage point.”

“It was a heavy, heavy weapon,” said Roy Broomfield, a patient advocate at the hospital. “It could have done some serious damage.”

The FBI said around 2:30 p.m., shots were first pumped into the hospital entrance on Taylor Street – shattering the glass. Shots from the assault rifle continued inside.

“All of a sudden, they start yelling, ‘This guy has a gun! He has a gun!’” said witness David Townsend. “I heard like some like fireworks. I thought it was a joke, like somebody shooting fireworks off.”

“I saw the guy run in there with a long, black — I don’t know what kind of shotgun it was — and he went into the pharmacy. And that scared the bejesus out of almost everybody that was sitting in the pharmacy,” said witness David Leslie. “I saw everybody hollering and running, and the security guards came in there and made them get down.”

“All of a sudden, we heard a couple shots, ‘Pow! Pow!’” Weems added. “And that people started scattering a little more. I ran through the building screaming for people to get out the building.”

Former Army medic Oliver Robinson came face to face with the shooter.

“I told him, ‘You don’t have to go through this. Just put that thing down.’ And by then, the VA police were coming down the hall and they drew weapons and took him down,” Robinson said. “He didn’t fight or anything.”

The shooter’s motive remained unclear Monday night. Patients described him as rambling, saying things like: “Who hit me? Who hit me?”

“He kept saying when he got inside the VA: ‘Where’s the ER? Where’s he ER?’” Townsend said.

The hospital said the man who fired the gun is not a veteran.

Nadeem Kahn, a worker in the radiology department, described the panic: “I saw more people running like crazy: doctors, nurses, everybody.”

“I couldn’t believe it. I said, ‘What is happening here? Are they kidding? Are they joking? What is going on?’ This is a VA — this is a federal building. How can that be possible?” Kahn said. “But then luckily no one was hurt. He just randomly shot a couple of shots.”

Video shows VA police officers arresting the suspect, dressed in a white t-shirt and baggy pants, inside the hospital. He apparently did not fire any shots inside. A weapon was recovered, police said.

Witness says shooting happened outside of the hospital. Shooter then entered hospital. No word on injuries @cbschicago pic.twitter.com/5xLeYNLeJN — Charlie De Mar (@CharlieDeMar) August 12, 2019

CBS 2’s Tim McNicholas tweeted video of the police response outside the hospital.

A chaotic scene outside the Jesse Brown VA Medical Center at Taylor and Damen after a report of shots fired. I am hearing there are no injuries, hospital has been shut down and one person is in custody. Waiting to learn more. pic.twitter.com/pG9eaHYfce — Tim McNicholas (@TimMcNicholas) August 12, 2019

“You know, we think about some of this nature all of the time because its volatile environment, and we hope that it doesn’t happen today,” Broomfield said. “Fortunately, no one was hurt. Security was on top of it. It was nice to see. And the response from CPD was fantastic.”

The hospital was shut down for some time following the gunfire incident, and workers were ordered to shelter in place. Police said the building was not evacuated.

Per CPD: Shots were fired in vicinity of Jesse Brown VA Medical Center—unknown if inside or outside Unknown if anyone shot. Somebody is in custody. We are on the way. Updates to follow. @cbschicago — Charlie De Mar (@CharlieDeMar) August 12, 2019

The FBI advised that anyone who was present during the gunfire incident and is in need of support should call the VA’s crisis line at (800)273-8255.

The shooter’s name has not been released. Chicago Police say they have had contact with him in the past.

The medical center, located at 820 S. Damen Ave., is a 200-bed acute care facility that provides care to approximately 62,000 enrolled veterans in the Chicago area.

A strikingly similar incident happened at the hospital a decade ago.

In September 2009, Kermit Washington walked into the lobby of the same VA Center and fired a shot. Nobody was hurt.

Washington was charged with killing his parents inside their home, and he then went to the hospital, where he barricaced himself inside an emergency exam room for several hours.