Police are exploring whether other Calgary foster children may have been subjected to sexual abuse following what are described as “horrifying” allegations against a 13-year-old.

Officers were called to a southwest foster home around 9:40 p.m. Tuesday by a witness with “direct knowledge” of a disturbing act taking place inside, said Det. Stephen Johnston.

Late Wednesday, police announced a 13-year-old boy living in the home had been charged with sexually assaulting and sexual interference against three other kids in the home ages six and under.

The alleged crimes were each committed separately but all took place on Tuesday, police believe.

Johnston said his child-abuse unit is often faced with difficult calls, but “This one, I daresay, is more shocking than most.”

Investigators have also learned other foster children previously lived in the home with the accused, who is being held in the Calgary Young Offenders Centre but cannot be named under provisions of the Youth Criminal Justice Act.

“We are looking into the possibility now that this was not the first incident that occurred,” Johnston said.

The Alberta government’s human services ministry oversees the operations of foster homes in the province.

Spokesperson Roxanne Dube-Coelho said a review into an alleged incident of this nature would be typical but could not speak further to whether concerns had been raised about the residence previously.

“I can tell you the younger children have been removed from the home,” she said. “They are in a safe placement.”