Cornered against the tiles of a Hercules high school locker room, the boy takes blows to the head as the camera catches the action.
Shouts of encouragement bounce off the walls. At least two boys take turns punching the boy in the face.
The assault, later posted on the Internet, is real: Attackers crushed a 17-year-old's jaw, a juvenile is under arrest and police are seeking a young man.
The group of males attacked the victim at the high school shortly before 1 p.m. May 6, said Hercules police Deputy Chief Tom Muehleisen.
The footage then appeared on a Web site, a fact the victim's mother called "really disturbing."
"That would be any parent's worst nightmare," said Lorena Cruz, the mother of Hassan Rahgozar, the beaten boy.
Youths recording such behavior for public dissemination is becoming more common, experts say.
Almost every unsavory adolescent behavior now has an online counterpart, said Nancy Willard, executive director of the Center for Safe and Responsible Internet Use, a consulting company based in Eugene, Ore. Part of the problem is the powerful sense of anonymity people feel the Internet affords.
Teens do things online they wouldn't want their parents to know about, said 64 percent of respondents ages 12 to 17 in a March study by the Pew Internet and American Life Project.
"One of the social norms for kids is what happens online stays online. If it happens online, nobody offline is supposed to know about it," Willard said.
"Kids think, 'On the Internet I have a free-speech right to post anything I want about anybody, without concern for the possible impact on that person.' "
Kids use camera phones to take surreptitious photos of other students in locker rooms, then post them online. They create message-board versions of "burn books" and use instant messaging to harass less popular peers.
The problem has worsened in the past few years, said Willard, as blogging, social networking and peer-to-peer sharing sites explode in popularity.
"The new information and communication technologies are putting the capability of publishing into the hands of kids, and kids do not have any understanding of the ramifications," Willard said.
"Life on the screen tends to be perceived as a game. Nothing is real, and anything using these technologies isn't real," Willard said.
The Web site where the beating footage appears allows people to share videos and photos free but bans pornographic, obscene or otherwise offensive material, reviewing files before they're uploaded for public view.
"We do like to run a clean and safe site, but we rely heavily on user input," said Gordon Page, managing director of PutFile Ltd., which is based in the United Kingdom.
Users can submit complaints, but Page said he was not aware of any complaints about the assault video.
"What some people find offensive may be different than what other people find offensive," Page said.
Police said they do not know who filmed the event or posted it.
"That's part of our ongoing investigation, too," Muehleisen said.
On the day of the attack, officers arrested one minor. He is at Juvenile Hall and is expected to be charged with felony assault today, Muehleisen said. Police think the other suspect may be someone over age 18 who does not attend the school, he said.
The attack seemed to be in retaliation for the victim giving information on the arrested juvenile, who had been suspended for an April 29 fight in which Rahgozar was also involved.