SEPTA police are on the lookout for a young man who knocked out a 60-year-old Market-Frankford El Line passenger on the 15th Street Station platform Monday afternoon.

Thomas J. Nestel III, SEPTA Police's chief, said Tuesday that the incident started inside a train car headed to Center City and escalated to the attack that happened at 3:15 p.m. on the westbound platform of the El's 15th Street Station.

SEPTA police released video of the incident with the hope of identifying the parties involved.

When boarding the train at an undisclosed location around 3 p.m., the victim, who wore a gray cap and tan jacket, stepped on the foot of a teen standing just inside the doorway, Nestel said.

The teen's friend, a young man between the ages of 17 and 19 and dressed in a blue hooded sweatshirt, became confrontational with the older passenger. Tensions escalated throughout the ride, according to Nestel.

The confrontation spilled out onto the 15th Street Station platform once the train stopped.

A group of teens surrounded the victim, and the young man swung from the side and hit the victim's face. The man appears to have been unconscious before he hit the ground, the video shows.

"He never saw it coming," Nestel said.

The victim suffered a broken jaw and an open wound that required stitches, Nestel said.

Nestel said he is not cooperating with authorities, so SEPTA police are asking for anyone with information to contact SEPTA Police at 215-580-8111.