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."

"The Northside Hospital family is grieving" in the wake of Wednesday's mass shooting in a waiting room at the Northside Medical Midtown office, Northside Hospital said in a statement on Thursday.

The hospital is encouraging people, wherever they are, to "take a moment or two" on Friday to "pause and reflect" in what the hospital is calling a "Day of Reflection."

"We deeply love and support the employees and patients who were directly involved, as well as others impacted in different ways," the hospital said. "Let us hope that this kind of tragedy never happens again -- not to us, not to anyone."

Hospital officials added, "We are incredibly grateful for the quick and courageous response of Atlanta-area law enforcement. Northside also thanks our colleagues at Grady Memorial Hospital for the care being provided to the victims. Their actions brought comfort and safety to our patients and staff at Northside Medical Midtown."

-ABC News' Will McDuffie