Officials say a mass fainting incident at a secondary school in Ripon, in northern England, on Wednesday was caused by a "ripple effect" after dozens of students fell ill.

At least one student fainted during a school-wide assembly — a moment of silence being held to commemorate Armistice Day — that then reportedly sent other students into a panic. Up to 40 students began feeling sick and some fainted, according to the BBC, triggering an emergency service response.

"We are of the view the children just fainted and there was a ripple effect throughout the school," Yorkshire Fire and Rescue Manager Dave Winspear told UK news outlet ITV. "More children felt anxiety and started to feel concerned and the thing has escalated."

North Yorkshire Fire and Rescue Service responded to reports just after 12 p.m. of students at the Outwood Academy in Ripon in Yorkshire feeling sick and fainting.

Some had to be seen by paramedics; at least two were brought to the hospital.

"When the first boy was sick, we thought it was one off but then there was a big slap on the floor and someone had fainted," an unnamed student told the BBC. "After that it was a bit of a domino effect, another three or four collapsed and then people started leaving the hall to get fresh air. One of the boys who fell was a bit concussed and had a big lump on his head."

But after inspecting the school, accompanied by hazardous materials specialists, emergency workers found no source for the mysterious illness.

"We have carried out a very thorough assessment and found no hazardous materials or anything untoward in the school environment," Winspear said.