After students got sick during a monthly Mass at the John Carroll School in Harford County on Monday, officials evacuated the building and closed the school on Tuesday. Eighteen students were evaluated at local hospitals.

School officials said in messages to parents they had ruled out “environmental contaminants” as a cause for the illness, but did not immediately specify what happened.

Wednesday afternoon, a spokesperson for the school confirmed the only symptom that staff or students reported was “lightheadedness.”

School spokesperson Kathy Walsh said no students were hospitalized overnight.

The school initially said two students “required medical attention” during the schoolwide Mass, and then a few other students began to feel ill. School officials ended the Mass and decided to dismiss students early. The school was evacuated for a “thorough inspection” from a hazmat team.

The school was closed Tuesday to allow for additional testing. Staff from the Harford County Department of Emergency Services went to the school around 6 a.m. Tuesday and found normal readings on “multiple” meters, ruling out an “environmental issue” as the cause of the sickness, an email from the school said.

Ronya Nassar, a spokesperson for the Harford County Health Department, confirmed there is no ongoing or public health risk related to the incident.

Classes resumed Wednesday and after-school activities resumed Tuesday afternoon.

Rich Gardiner, a spokesperson for the Bel Air Volunteer Fire Company, which responded to the John Carroll School on Monday, said as far as he knew, there were no other specific symptoms reported or seen at the school.

The John Carroll School is a Catholic, co-educational high school in Harford County. It first opened in 1964.