BILLINGS, Mont. (AP) — Attorneys for a group of Montana landowners, health advocates and conservationists have filed a lawsuit seeking to force energy companies to divulge more about the chemicals they use in energy production.

Oil and gas industry regulators in September denied a petition to make companies disclose the details on chemicals used during hydraulic fracturing, or "fracking."

A lawsuit challenging that decision was filed Tuesday in state court in Lewis and Clark County.

During fracking, companies pump industrial fluids underground to release oil and gas trapped in rock formations. Harmful chemicals in those fluids can threaten water supplies.

Under a 2011 state rule, companies have the discretion to conceal the ingredients of chemicals they consider trade secrets.

Plaintiffs' attorney Katherine O'Brien with Earthjustice says the trade secrets exemption violates the Montana Constitution.