CHICAGO — A 14-year-old boy has been arrested in the sexual assault of a 15-year-old Chicago girl that was streamed live on Facebook and watched by about 40 people who didn’t report it to authorities, police said.

Police spokesman Anthony Guglielmi said more arrests are expected as the investigation continues. Investigators have said the attack involved five or six males.

Guglielmi said the juvenile suspect faces felony charges of aggravated criminal sexual assault, manufacturing of child pornography, and dissemination of child pornography.

Police said they were not aware of the mid-March attack until the girl’s mother approached Police Superintendent Eddie Johnson as he was leaving a police station and showed him the video.

Johnson was ‘‘visibly upset’’ after he watched, both by the content and the fact that no one notified police.

The girl’s mother said last month that her daughter received online threats after the attack and that neighborhood youths were ringing her doorbell looking for her. The girl’s family has since relocated.

The video marks the second time in recent months that Chicago police have investigated an apparent attack broadcast live on Facebook. In January, four people were arrested after video showed them allegedly taunting and beating a mentally disabled man.

ASSOCIATED PRESS