Police have launched an investigation after a man was caught on video knocking out an older man at the Chicago Avenue Red Line stop while others laughed and mimicked the attack.

The assault occurred in April, but the video has only recently surfaced on the Internet.

The video, posted at worldstarhiphop.com, shows a man in a tan coat moving among people on the southbound subway platform as a group of youths joke and laugh. One of the youths starts following the man, turning around and smiling as his friends whoop and laugh.

The youth, dressed in a black vest jacket, then appears to tap the man to get his attention. When the man turns around, the two appear to exchange a few words before the youth suddenly strikes him on the side of the head.

The older man falls on his back so hard his hat flies off, according to the video. The youth and his friends board a waiting train as a woman kneels down and touches the man and makes a phone call.

As the train pulls out, one of the youths can be heard saying, "Who just did that?" Another responds, "He laid the (expletive) out," and "He knocked his ass out." One of the youths then mimicks the punch.

Police and paramedics were called to the station and the older man was taken toNorthwestern Memorial Hospital, where he was treated and released, according to Police news Affairs Officer Robert Perez.

While some websites have named the youth in the video, he has not been charged.

Police are seeking the man who was hit to see if he wants to pursue charges. They asked anyone who has seen the victim or has information on his whereabouts to contact police.