In Washington, two people were randomly punched in separate incidents but suffered only minor injuries and did not lose consciousness, while two similar assaults in Philadelphia also have police on alert.
In September, Ralph Santiago, 46, of Hoboken, N.J., died from injuries resulting from a suspected knockout attack that sent him careering backward into a fence, where his head got lodged. Three teens -- two 13-year-olds and a 14-year-old -- have been charged with murder.
In late May, two teenagers admitted to fatally beating and killing a man in Syracuse, N.Y. They admitted that the attack began with the intention of knocking out the victim, Michael Daniels, with a single blow. Both teens, 16 and 13, were sentenced to 18 months in jail.
And earlier in May, Elex Murphy, now 20, was sentenced to life in prison plus 25 years in St. Louis for killing a Vietnamese immigrant as part the game in 2011.
Surveillance footage has also gone viral of another attack from 2012, in which a 50-year-old Pittsburgh, Pa., English teacher named James Addlespurger is struck and falls limply to the curb.
Experts say the violent acts appear to be driven in part by a pack mentality and peer pressure put on the perpetrators.
"These kids have effectively de-humanized others," Former FBI special agent Brad Garrett told ABC News. "They are being drastically influenced by the groups to commit the acts."
The Associated Press contributed to this report.