MILWAUKEE (CBS Local) — A 10-year-old girl was injured when a janitor’s gun accidentally discharged at her school, authorities in Milwaukee said.

Police say the janitor brought a firearm inside St. Josaphat’s Parish School on April 5, which discharged into a wall. Debris from the wall hit the student, causing a bruise.

The victim’s mother, using her niece as a translator, described what happened.

“She says it was a nail that had come out from the air conditioning that wasn’t on right,” the niece told WISN. “That’s what came flying out and hit her.”

The victim’s mother said school officials did not mention the gun until 12 days after the incident, when she was asked to speak to police.

Milwaukee Police say they were not notified about the incident until April 17, CBS affiliate WDJT reported.

The janitor was arrested and criminal charges are expected.

“An internal investigation was commenced, and the employee has been terminated,” St. Josaphat Parish said in a statement.