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 suspected gunman, 24-year-old Deion Patterson, was armed with a semi-automatic handgun when he allegedly shot five people in a medical center waiting room, according to court documents.

The documents identify the surviving victims as Georgette Whitlow, Lisa Glynn, Jazzmin Daniel and Alesha Hollinger. All four of them remain hospitalized.

Hollinger was shot in the face, according to the documents, while Daniel was shot multiple times in the abdomen. Whitlow was shot in the arm and Glynn was shot in the abdomen.

Amy St. Pierre, 39, an employee of the Centers for Disease Control and Prevention, was killed in Wednesday's shooting.