More than a dozen students and staff were sickened by an unknown irritant at Ballenger Creek Middle School in Frederick, Maryland, on Friday.

The school was evacuated at around 10:15 a.m. while a county HAZMAT team was called to the scene after staff reported a strange odor.

The Frederick County Sheriff’s Office said the HAZMAT team detected an unknown irritant released inside the school. Five students and one staff member were taken to Frederick Health Hospital for evaluation. Seven other students and one staff member were evaluated on the scene for minor injuries.

Students were let back into school for the rest of the day.

The Frederick County Sheriff’s Office is investigating the incident, including reviewing video footage from inside the school, and said that once it identifies a suspect or suspects, criminal charges will be forthcoming.