HANOVER, Pa. (AP) — Authorities say they still don’t know what caused several dozen children and adults to fall ill at an eastern Pennsylvania school last week, prompting an evacuation.

Chief Scott Van Why of the Hanover Township Volunteer Fire Department told The (Allentown) Morning Call on Sunday that tests of the air turned up nothing to explain what affected 48 children and adults Friday at Lehigh Valley Academy Regional Charter School.

Emergency responders were sent to the school after nearly a dozen people were reported sick in one of the three buildings. Officials said that building, which houses seventh- through twelfth-grade students, was evacuated “out of an abundance of caution,” but normal operations continued at other buildings where younger students are taught.

Susan Mauser, CEO of the Lehigh Valley Academy Regional Charter School, said most of those taken to hospitals for evaluation had been released as of Friday night, LehighValleyLive.com reported.

Fire and hazmat officials checked for oxygen, carbon monoxide, hydrogen fluoride and flammable gases and found all within normal ranges, Mauser said. A visual inspection for hazardous materials also failed to turn up anything and the HVAC unit was found to be operating properly, she said.

The building owner was scheduled to bring in air quality specialists in coming days to further evaluate the building, Mauser said.

The charter school has 1,825 students who come from 16 school districts in the area.