Officials said a very small amount of natural gas escaped into Lake Zurich's Middle School South ventilation system Tuesday.
Lake Zurich firefighters were dispatched around 2 p.m. Both Middle School South and Isaac Fox Elementary School had been evacuated, said Capt. Dave Borst.
Borst said a person at the scene reported an odor of natural gas was noticed near the kitchen at Middle School South.
Officials discovered work was being performed on a rooftop ventilation unit and when that unit had been turned back on, a trace amount of gas escaped.
Borst said firefighters did not smell an odor nor detect natural gas.