Police are searching for a man who allegedly punched a five-year-old in the face Saturday on a New York City subway.

The New York Police Department (NYPD) told Fox 5 New York that the incident took place before 4:30 p.m. Saturday while the boy was riding a southbound “G” train with his mother when the stranger allegedly socked the boy in the face for no reason.

The man then allegedly taunted the child before fleeing the scene, police said.

The boy suffered from bruising and swelling on the left side of his face. Emergency medical personnel took him to NYU Langone Health Cobble Hill, where he was treated and later released.

Authorities are searching for a 20-year-old black man about six feet tall and 160 pounds with short black hair, a mustache, and a medium build. He was last seen in public wearing an olive green shirt, black jacket, beige cargo pants, and black sneakers.

These subway attacks are not uncommon. In December, a suspect allegedly punched a 70-year-old on a New York City subway stopped in Queens in an unprovoked attack.

The attacks are reminiscent of a dangerous trend called the “knockout game”—where teens randomly walk up to a stranger and try to knock them out with one punch.

The “game” began in areas of New York and New Jersey but spread to other cities such as Washington, DC, St. Louis, and Pittsburgh as more people viewed the disturbing trend on social media.

Breitbart News reported that the trend took hold in areas where guns are heavily regulated and citizens are less able to defend themselves.