CHICAGO (CBS)–An 18-year-old is among four people now charged in a mob attack on the CTA Red Line.

Michael Sardin is being held on $5,000 bond for felony aggravated battery and mob action.

(Chicago Police)

Police say he punched and kicked two people multiple times at the Chicago Avenue Red Line station on Dec. 29.

A group of eight to ten other people also attacked the victims, according to police.

One of the people who were attacked sustained “great bodily harm,” according to police. He is recovering from bone fractures, abrasions and lacerations, police said.

A video posted to Facebook last week shows one of the victims being beaten by the mob of young men.

Sardin later turned himself in to police.

Investigators credit the high-definition cameras installed at the CTA station for catching the four people charged in the attacks.

The CTA recently added more than 500 cameras around the Red, Blue and Brown Line trains to improve safety.