A janitor discovered six bales of hay, doused with either gasoline or oil, stacked against the rear entrance of a Billings middle school.

The Billings Gazette reports gas cans were found scattered around the hay bales on the receiving dock at Castle Rock Middle School Thursday morning.

Principal Shaun Harrington called the police and fire departments. Agents from the federal Bureau of Alcohol, Tobacco, Firearms and Explosives also were brought in to investigate at the school in the northern part of the city.

Harrington says agents swept the school before classes started to make sure the building was safe. Nothing was found.

Officials are still investigating.