A 4-year-old girl was sexually abused last month at an elementary school in Avondale on the Northwest Side, according to Chicago police.

The girl went to the bathroom about 7 a.m. Sept. 16 when she ran into a man inside, police said. The man touched her inappropriately, then made her touch him.

Police sources said the incident happened at a school in the 3600 block of West School Street. Reilly Elementary School is located at 3650 W. School St.

In a letter shared with the Reilly Elementary community a few days later, Principal Ken Fitzner said that the Office of the Inspector General was investigating an “alleged incident of inappropriate conduct between a student and an adult...”

“Please know that we are taking this situation seriously, and we remain committed to providing our students with a safe, positive learning environment where they can reach their full potential,” Fitzner continued.

No one is in custody, police said.

A spokesman for the Illinois Department of Children and Family Services said the agency was investigating the incident, but declined to comment further.