Images of an apparent "happy slapping" attack showing a victim being kicked and beaten, allegedly in Birmingham, has been posted on the internet.
The video has been described as "sick"
The 80-second video, put on the YouTube website by someone called 'Brummie', appears to show an argument between two teenagers followed by an assault.
West Midlands police say they are taking the pictures seriously.
Detectives are appealing for the victim to come forward and want to know where the attack was filmed.
The video has attracted a range of comments from people who have watched it online.
One has described the footage as "sick" while another congratulated the attacker on a "nice uppercut".
Prof Kevin Browne, a forensic psychologist at the University of Birmingham, said: "It was only a matter of time before someone started using this sort of equipment to film what is abhorrent and place it on the internet as some sort of trophy."