A New Hampshire school where 22 students and staff were sickened has reopened as firefighters monitor school equipment to figure out the cause.

The group at St. Thomas Aquinas School showed symptoms of carbon monoxide poisoning Wednesday and the school was evacuated.

However, when firefighters tested the air, no carbon monoxide was detected. Since then, eight carbon monoxide detectors have been installed at the school.

Derry Fire Capt. Scott Haggart says further monitoring Wednesday night and Thursday morning has not produced an answer on what caused the illnesses.

The students and staff reported feeling faint and experiencing headaches and nausea. They were treated and released from hospitals.