Students at Roosevelt High School returned to class Tuesday morning, three days after a pepper spray incident sent more than a dozen people at the school to the hospital.

According to school officials, on Friday someone released pepper spray inside a fourth-floor cafeteria, leaving 12 students and two staffers coughing, gasping for breath and feeling dazed. Most of the victims of the attack have since recovered.

Police and school officials are still trying to determine who brought the pepper spray into the school.