YARMOUTH – Yarmouth High School on Tuesday reopened following an evacuation the day before.

The evacuation occurred after three students lost consciousness within 30 minutes of each other Monday morning, before other students reported feeling ill.

Superintendent Andrew Dolloff said officials are investigating whether an external source was the culprit.

“One of the students had some carbon monoxide in their blood, another didn’t have any at all and the third had a trace amount ...” Dolloff said.

Yarmouth Fire Department tested the school for a variety of contaminants, including carbon monoxide, but found nothing dangerous, according to Dolloff.

On Tuesday, Dolloff said precautions have been put in place following this incident.

“What we’ve done so far today is that we’ve tested the building again this morning before students came,” Dolloff said. “We changed every air filter in the building overnight and through the morning hours so we can provide students with relief that we’re looking out for their safety.”

The superintendent said he would meet with local epidemiologists and doctors Tuesday afternoon to prevent similar incidents.