Three boys have been arrested in connection to a video showing a senseless attack on a mentally-challenged man in Germantown, Pennsylvania.

In the Facebook video, a boy is seen sucker-punching the 38-year-old victim as two others watch and laugh.

The disabled man staggers back in shock and pain before a second boy appears to lunge at him for another attack.

Philadelphia Police said that three suspects, who range in age from 12 to 15, were taken into custody Tuesday from school. Authorities are still searching for a fourth suspect, who is 15-years-old, according to news station WPVI.

“Obviously, the video is very disturbing. It appears this male who is 38-years-old was targeted because he is mentally challenged. It’s a senseless act of violence,” Philadelphia Police Captain Malachi Jones told WPVI.

The video, which was viewed more than 1.5 million times, has since been removed.