CHICAGO (CBS) — A Chicago police officer is under investigation, after hitting a teenager in the head while holding a pair of handcuffs last week at the Roosevelt subway station on the Red Line.

Two cell phone videos posted on Facebook captured the arrest now sparking outrage.

In one clip, two officers can be seen detaining a teenager, while he and his friends keep asking why. The officers don’t say.

A second clip seems to begin moments later, after more officers have arrived. One of the officers is seen repeatedly striking the teen in the head, with a pair of handcuffs in his hand.

As the crowd on the platform shouts at police, that same officer can be seen backing them off, still holding the cuffs in a raised clenched fist.

The teen is seen being led up the escalator, after struggling to get his footing as he was picked up off the platform.

Police Supt. Eddie Johnson said officers had detained the teenager, because he matched the description of a robbery suspect.

“We had a crew of young individuals going around on the Red Line robbing people, and he was identified as a possible suspect with that particular group. So that’s why they were approaching him,” Johnson.

The teenager later was released without charges.

“He wasn’t charged that night, because by the time the dust settled on that particular robbery, the victim had wandered off. So we still haven’t located that victim yet,” Johnson said.

Police said the incident on Nov. 25 is under investigation by the Civilian Office of Police Accountability.

“This was an incident where an individual was resisting arrest. A use of force investigation has been opened into the Officers actions by the Civilian Office of Police Accountability (COPA) to establish if policies were violated and CPD will fully cooperate with COPAs investigation,” Chicago Police Department spokesman Michael Carroll stated in an email.

The Civilian Office of Police Accountability confirmed they’re investigating the use of force by the officer.

The Chicago Police Department said it’s cooperating with COPA’s investigation, and said officers will be disciplined if it is determined they violated department rules.

“If it’s determined that he used use of force incorrectly, then we’ll deal with it,” Johnson said. “If there’s something that we can do better, or if an officer needs to be held accountable, then that will occur.”