A south suburban man was released Sunday after turning himself in to police the day before in connection with the beating and robbing of an off-duty Chicago police sergeant in the Loop on Thursday, reports the Chicago Tribune.

Brian Williams, 21, of Hazel Crest, was one of four people who were held in connection with the incident, according to a law enforcement source. Williams was released from Area Central detectives headquarters about 2 p.m. Sunday, said his attorney, Max Solomon.

The altercation with the off-duty sergeant occurred at 3 p.m. Thursday in a downtown parking garage. Chicago police said the sergeant, who was wearing civilian clothes, and two other men were exiting the elevator on the 12th floor of the garage and walking toward their car when Williams and at least five other people confronted him.

Police reported several members of the group hit the sergeant and knocked him to the ground. When the sergeant's colleagues intervened, the attackers fled in a Buick Rendezvous after taking the sergeant's gun, police said.

The sergeant was taken to Northwestern Memorial Hospital in serious condition, suffering facial fractures, a concussion and bleeding in his brain, police said.

Before surrendering to police Saturday afternoon, Williams said the sergeant instigated the fight by using a racial slur, and that his friend was the only one who attacked the officer.