PITTSBURGH (AP) — A teenager who sucker punched a schoolteacher in a downtown alley — an attack caught on surveillance video — apparently picked the victim at random, police and school officials said.

The attack happened last Thursday at about 3:30 p.m. The surveillance video, which Pittsburgh police used to make an arrest Tuesday, shows the 15-year-old suspect walking with several other youths past 50-year-old James Addlespurger. Addlespurger, who teaches English at an arts high school, was walking in the other direction and did not appear to interact with the youths in any way.

The suspect suddenly approached Addlespurger and punched him in the head. The teacher fell hard onto a curb as the teen and the others continued walking.

"We don't know the mind state that that young man was in, but there's something very terrifying about it," said Pittsburgh Councilman R. Daniel Lavelle, whose district includes downtown. "In the video it appears he was laughing afterwards, which is very disturbing."

The suspect was taken to a juvenile detention center on a charge of simple assault but was not identified because he's charged in juvenile court.

City schools spokeswoman Ebony Pugh said the school system can't take any formal action against the suspect because the attack happened off school grounds and after the school day had ended.

The suspect attends a school for troubled youths in another part of the city. Police said there's no evidence the suspect, or his friends, knew Addlespurger or exchanged words with him.

Addlespurger didn't immediately return a call for comment, but has said on Facebook and in brief comments to reporters that he's recovering and that "the video pretty much speaks for itself."