CASPER, Wyo. (AP) — Oil field crews in Natrona County still have not found the source of a chemical odor that forced the closure of a nearby school.

The Casper Star-Tribune reports Fleur de Lis Energy, which operates the Salt Creek oil field, initially focused its efforts on a series of plugged wells near the school and is expanding the search area from there. A buildup in pressure could be causing a leak.

Midwest School with about 150 students in grades K-12 closed Thursday after staff detected a gas odor. School officials said the students would finish the last few days of class at another location in Casper.

State regulators say there is no threat to public safety, but they continue to monitor air quality.