College of West Anglia 'irritant': 'Pepper spray' likely cause
A pepper spray was the likely irritant that led to 30 people at a Norfolk college needing medical treatment, the college said.
Emergency services were called to the College of West Anglia in King's Lynn on Friday.
An adult student was taken to hospital as a precaution but was later discharged.
Principal David Pomfret said he would be leaving "no stone unturned" to find out what happened.
Emergency services including a Magpas helicopter treated people at the site for breathing difficulties and watery eyes.
A number of classes at the Tennyson Avenue campus had to be cancelled but the full college has since reopened.
A college spokeswoman said emergency services believed the likely cause was "the introduction of a substance to the third floor area by a person or persons as yet unknown".
"Discussions with the emergency services suggest that this is likely to have been a CS-type substance, mace or pepper spray, but this has not been confirmed," she said.
"It is being treated as a criminal act and the college will be working closely with the police to identify the culprits," she added.
Mr Pomfret said: "I would like to reassure students and staff that we have already started an internal investigation to support the police and will be leaving no stone unturned to ensure this matter is brought to a speedy conclusion."
Norfolk Police confirmed it was investigating the incident but has yet to comment further.