The dangerous "knockout" attacks on strangers in large U.S. cities are leading to arrests, more officers flooding the streets and more warnings for vigilance among an unsuspecting public.
Hoodlums have dubbed the violent practice as the 'Knockout Game,' where teens try to randomly knock out strangers with one punch.
The attacks have raised concerns across the country. Recent attacks have occurred in New York, New Haven, Conn., Washington, D.C. and suburban Philadelphia. But the violent attacks go back several years. In 2011, St. Louis, Mo. had a rash of incidents, one of which led to the killing of a Vietnamese immigrant.
The trend may have hit the west coast with an attack Tuesday on a man in downtown San Diego, according to local news stations.
Some of the assaults are recorded and posted on social media by the attackers.
In New York, where there have been seven incidents in the last month, police arrested four suspects Friday in Brooklyn, NBC News reports.
The NYPD told the station that a 24-year-old victim was walking in the Borough Park section of Brooklyn when he heard people talking about the game. The man was then hit in the head.
As the attackers ran away, police officers helped the man and fanned out across the area to find the suspects nearby, NBC reports. Their names were not released.
New York police Sgt. Brendan Ryan did not respond to a request for information about the arrests, but said in an email that extra officers have been assigned to the Crown Heights area of Brooklyn for the immediate short term.
The attacks in New York have racial overtones because the attackers are black and the victims have been Jewish.
In New Haven, police spokesman David Hartman says police are investigating six incidents in the last month as part of the knockout trend. He says investigators have identified three people of interest, though no arrests have been made.
He says no one suffered serious injuries in any of them. But as a result of the attacks, he says, more undercover officers are patrolling downtown and a neighborhood called South Hill, where five of the assaults occurred.
"I think the fact that this has been labeled a game is sickening," Hartman says. "This is not a game. This is violent."
Will Marling, the executive director of the National Organization for Victim Assistance, says this trend is not an epidemic.
"But it could be the start of one," he says, because the attacks have a social media component that could go viral. "As experience shows, other kids will see this is an easy thing to do and then it becomes group think."
He says the attacks are an example of why there is a need for a deeper conversation with young people about respect.
Michelle Boykins, a spokeswoman for the National Crime Prevention Council, says what is so disturbing about the trend is that it is so random and the intent is to hurt someone seriously.
She says that the instances often involve someone walking alone, so she suggests the tried-and-true ways to stay safe: walk with a friend and always remain aware of your surroundings.
"There is safety in numbers," she says. "And if you are by yourself, there is nothing wrong for you to decide to cross the street if you see a group of people walking toward you."