As Atlanta police search for the suspected gunman, 24-year-old Deion Patterson, they say a car was stolen "not long after the shooting" and has since been recovered.

"We are working to locate the carjacked vehicle and to determine whether it is connected to the shooting," police said.

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."

Police in Cobb County said Deion Patterson was seen on video in the county around 12:30 p.m. ET, roughly 20 minutes after the shooting, though it's unclear if he's still in the area.

Cobb County Police Sgt. Wayne Delk said during a press briefing that after learning Patterson might be in the area, his team checked flock cameras and "did discover that the vehicle had entered Cobb County," located northwest of Midtown Atlanta, where the shooting occurred.

"Around 12:30 p.m., we did spot what appeared to be a suspect in this area, which is why we have such a heavy presence," Delk said.

Police did not discover the video footage until around 2:30 p.m., and Delk acknowledged it's unclear if Patterson is still in the county.

"He might still be in the area," he said.

The vehicle Patterson was believed to be driving was recovered by Atlanta police in a parking garage, Delk said.

-ABC News' Will McDuffie