Pittsburgh police are investigating a random assault on a teacher last week that was caught on surveillance tape.

James Addlespurger was walking in a downtown alley as six young men were walking in the other direction.

You can see from these still pictures...one of the men punched Addlespurger in the face.

According to the Pittsburgh Post-Gazette, the blow knocked him down and he hit his head on a curb.

Three people passing by stopped to help Addlespurger as he lay on the ground.

He was taken to a nearby hospital with cuts and bruises.

Addlespurger said he didn't recognize any of the kids in the group.

Police have identified three of the teens seen in the video, but not the one who hit the teacher.