New York City police say a 16-year-old girl has been arrested and others are being sought after an assault captured on video showed a group attacking a 15-year-old classmate.

The video shows a group attacking the teenage girl at a McDonald's in Brooklyn on Monday while a dozen onlookers cheer. Police say McDonald's workers called 911, but apparently no one in the crowd tried to help.

NYPD Chief of Detectives Robert Boyce says Thursday that one girl was arrested. Her name wasn't released. She attended the same high school as the victim, who suffered black eyes and bruising, police say.

Police say the victim and other witnesses were refusing to speak to police.