Students are evacuated to the football stadium after the shooting at Apalachee High School in Winder on Wednesday. AP

In addition to the four killed, dozens of people were injured in the shooting at Apalachee High School in Winder, Georgia, CNN said, citing unnamed law enforcement officers briefed on the incident. Not all injuries appeared to be gunshot wounds, but rather also people hurt while fleeing.

One suspect was in custody, the Barrow County Sheriff's Office said in a statement. "What we see behind us is an evil thing today," Sheriff Jud Smith said during a brief news conference on school grounds.

Smith would not confirm that people were killed, saying only there were "multiple injuries" in the shooting.

The incident appeared to be under control and students were being released at midday, a Barrow County Schools spokesperson said.

ABC News quoted a witness, student Sergio Caldera, as saying he was in chemistry class when he heard gunshots. Caldera, 17, told ABC his teacher opened the door and another teacher ran in to tell her to shut the door "because there's an active shooter."

As students and teachers huddled in the room, someone pounded on his classroom door and shouted several times for it to be opened. When the knocking stopped, Caldera heard more gunshots and screams. He said his class later evacuated to the school's football field.

Live aerial TV images showed several ambulances outside of the high school. CNN said it witnessed a patient being loaded into a medical helicopter that had landed at the school.

The sheriff's office said, "multiple law enforcement agencies and Fire/EMS personnel were dispatched to the high school in reference to a reported active shooting."

The FBI field office in Atlanta dispatched agents to the high school to support local law enforcement, said Jenna Sellitto, a spokeswoman for the office.

The White House said in a statement that President Joe Biden had been briefed on the shooting "and his administration will continue coordinating with federal, state, and local officials as we receive more information."

Reuters