LAKEWOOD, Colo. (CBS4) – Investigators believe they have found the source of allergic-like symptoms that forced an evacuation of a Jefferson County school Monday and sent children to the hospital.

Jeffco Open School remained closed on Tuesday as the investigation continued. Thirty students and one assistant teacher suffered from symptoms of eye injuries, skin irritation and pain at the school. Seven of those had to be transported to nearby St. Anthony and Lutheran hospitals.

“In searching the playground area, employees found pieces of approximately six habanero peppers scattered in the wood chips,” Melissa Reeves with Jefferson County Public Schools said in a statement. “Coming in contact with the pepper oil would cause many of the symptoms that students experienced.”

It’s not clear how the peppers got on the playground.

Monday afternoon St. Anthony Hospital tweeted that their patients were all in fair condition.

Firefighters were first alerted to the toxic substance around 12:45 p.m. Monday. Dozens of children had to be decontaminated as a precaution.

Firefighters say their monitors were able to pinpoint the substance to the school’s playground. Firefighters were not able to identify the source but they were able to rule out some other possible causes.

“District teams are in the process of washing the playground equipment and wiping down surfaces inside to ensure that the school is free of any residual pepper oil,” Reeves said. “The wood chips in the area where the peppers were found are being replaced.”

“I think it’s hard to speculate where they came from and who might have done that,” Principal Scott Bain said.

Hot chilies are an ingredient in pepper spray that police use to subdue unruly people.

Classes will resume Wednesday morning.