At least 23 people were injured Saturday night when someone sprayed pepper spray during a family party on Chicago’s Southwest Side, according to authorities.

Police said the group was gathered outside at a backyard party around 11 p.m. in the 5000 block of South Rockwell Avenue in the city’s Gage Park neighborhood when someone sprayed pepper spray into the yard from a nearby alley.

Fire officials said it wasn’t immediately clear what the spray was, but that it was a “foreign substance.”

Chicago Fire Department spokesman Larry Langford said 23 people, many of them children, were injured in the attack and transported to area hospitals.

The victims were treated and released from various area hospitals, according to Chicago Police News Affairs Officer Amina Greer.

Greer said no one at the scene was able to give a description of the suspect.

The Chicago Tribune reports the group was gathered for an anniversary party. A couple was celebrating 50 years of marriage with family and friends.