A frightening new trend among teenagers is sweeping the nation.

It's called the 'knockout game' and features people walking up to random people on the street and knocking them out with one punch, sometimes killing them. Women have even been among the targets.

It's growing in popularity in several cities, with New York leading the charge, thanks to seven incidents. Knockouts have also been reported in Pittsburgh,St. Louis, Chicago, Syracuse and Hoboken, NJ.

It's unclear if there's a coordinated effort or to attack victims based on race, religion or any other characteristic or it they are random stand-alone attacks. It's also not known just how widespread the problem is.

New York City Police Commissioner Ray Kelly says this is an issue that he has seen before, noting, "We've seen this in the past but I would like to urge anyone who has been victimized by this to come forward.