At least four people were killed and 16 others were injured after gunfire erupted outside a crowded bar on St. Helena Island, South Carolina, early Sunday morning. The Beaufort County Sheriff’s Office said the shooting happened around 1 a.m. at Willie’s Bar and Grill, located about an hour north of Savannah, Georgia.

ABC News reports that when deputies arrived, they found a large crowd and several people suffering from gunshot wounds. Four victims were pronounced dead at the scene, and others were rushed to nearby hospitals. Officials later said 20 people were treated for injuries, including those who had died.

Authorities believe hundreds of people had gathered at the bar for a local high school alumni celebration when the shooting started. Witnesses said many ran for cover or into nearby businesses to escape the gunfire. Four people remain in critical condition.

Bar owner Willie Turral told local station WJCL that the violence broke out just after what had been a joyful evening. He said the shooting happened outside, but the sound of “machine gun fire” could be heard from inside. He described seeing people rushing for safety and bodies on the ground.

Turral said one of the victims was a security guard he knew personally, making the tragedy even more painful. He added that he had hired professional security for the event, which drew a large crowd of alumni and community members.

Local representative Nancy Mace called the attack “heartbreaking” and urged anyone with information to contact investigators. The Beaufort County Sheriff’s Office said in a statement it is questioning a person of interest but has not yet announced any arrests. Officials called the shooting a devastating event for the small island community and promised a thorough investigation.