An off-duty police officer is under investigation after they allegedly left a gun behind in a school bathroom that was discovered by three young students.

The incident happened on March 6 at about 2:30 p.m. at Catherine Cook School when three young girls told their teachers that they found a gun when they were inside a girls restroom, according to ABC News Chicago station WLS-TV.

Police said that school security responded to the scene and secured the firearm but, after a police report was filed, authorities discovered that the gun actually belonged to an off-duty Chicago police officer who worked as a security guard at the private institution.

An internal affairs investigation was opened at the school and, according to authorities, the officer will likely face some kind of discipline but will most likely not face any criminal charges in the incident.

The three girls who found the gun in the bathroom at the kindergarten through eighth grade school were unharmed and school officials could not immediately be reached for comment.

The police officer who is allegedly involved in this incident has not yet been identified.