A violent craze called the "knock-out game" is spreading from New York to other American cities as gangs of teenagers and young men sucker-punch passers-by on the street for entertainment.
The brutal phenomenon is fuelling racial tensions as most assaults have reportedly been conducted by black attackers on white victims.
In New York, the recent victims have all been Orthodox Jews. In online postings, some attackers have referred to their activity as "polar bear hunting", a reference to the skin colour of their targets.
The point of the "game" is to knock out the unsuspecting with a single punch. None of the victims have been robbed. The attacks are often captured by the assailants on mobile phone cameras and posted online.
Similar attacks have been reported in cities including Philadelphia, Chicago, St Louis and San Diego, apparently fueled by the internet videos. At least two victims have died from their injuries.
In Philadelphia, Jim Addlespurger, a teacher, was walking home in broad daylight when a group of teens passed him and one hit him without any warning.
"I was hit with one punch that knocked me to the cold concrete," recalled Mr Addlespurger, who has since watched the attack on CCTV footage of the street.
One chilling video posted online shows a young man in Washington running up to a woman from behind and hitting her with a sucker-punch so ferocious that she falls to the ground motionless. In another incident, in New Jersey, the suspects can be seen laughing and bragging about their attack. (© Daily Telegraph, London)