Dr. Ahmed Ali, a radiation oncologist at the building, told ABC Atlanta affiliate WSB that he was returning from his lunch break when he saw armed officers swam the scene.

"I was worried what was happening," he said. "Building security … they told me gunshots were coming from the 11th floor. They said there was a pool of blood in the elevator shafts when the doors opened."

-ABC News' Ivan Pereira

Of the four people who were shot and injured, three are in critical condition and one is in the emergency department, according to hospital officials.

All four are adults, officials said.

One person was killed and four were injured in the shooting at Laureate Medical Center in Atlanta, according to police.

"Officers are actively searching for the suspect and any other victims," police said.

Northside Hospital tweeted that it's "cooperating with law enforcement following the shooting at our midtown #Atlanta location earlier this afternoon. We urge people in the area to shelter in place and follow instructions from law enforcement on the scene."

Three of the four women who were shot and injured remain in the intensive care unit, Dr. Robert Jansen, the chief medical officer at Grady Health, said at a news conference Thursday.

Two of those three women are "critical" and will return to the operating room for additional procedures, he said. The third patient in intensive care is stable and hopefully can move out of the ICU later in the day, he said.

The fourth woman who was shot is in stable condition, he said.

"As of right now everybody is as stable as they can be given the nature of their injuries," Jansen said.

Jansen said he's spoken with two of the injured victims.

"They have been traumatized," he said.

"They are very grateful for the support and care they've received," he said, however, "the fact that they were in a health care facility just makes it worse."

Jansen noted that "the impact" of this shooting "on the staff is something you can't imagine."

"Unfortunately, in this day and age, seeing shooting victims is almost a daily occurrence here," he said.

"But when they come in like this, particularly because it happened in a health care setting, it resonates more," he said, adding that the hospital staff is being offered support.