BISMARCK, N.D. — An oil executive from Kalispell accused of dumping toxic drilling liquid and endangering drinking water for a county in southwest North Dakota has reached an undisclosed plea deal with federal prosecutors, authorities say.

Executive Drilling LLC President Nathan Garber is slated to be sentenced Sept. 26 in Bismarck federal court. Garber is charged with multiple felony counts of violating federal environmental laws. He faces more than 50 years in prison.

Terms of the plea deal and sentencing recommendations were not disclosed in court papers. North Dakota U.S. Attorney Timothy Purdon confirmed the plea deal on Thursday but would not elaborate.

Garber could not immediately be reached by The Associated Press for comment. A Bismarck attorney who represents Garber, Monte Rogneby, did not return telephone calls.

The criminal charges against Garber are connected to a state case against Halek Operating ND LLC, which was fined a record $1.5 million for putting drinking water at risk by illegally dumping more than 800,000 gallons of salty, oilfield wastewater into a former oil well in Stark County and then attempting to cover up the crime.

Garber ordered the illegal dumping and told workers to tamper with the disposal well southwest of Dickinson so it would pass a state inspection, court documents say.

Companies commonly dispose of the oil production byproducts by injecting them into an approved underground facility. Halek Operating owner Jason Halek admitted illegal dumping in court records but said that his company had transferred ownership to Executive Drilling and therefore wasn’t at fault.

State officials said the $1.5 million fine was the largest regulatory penalty ever for violating the state’s waste disposal rules, and Garber was the first person prosecuted for allegedly breaking them.

Garber was fined $2,500 in state court and given a two-year suspended jail term. North Dakota regulators said at the time that the penalties sent a strong message about the state’s stance on protecting the environment.

Federal authorities later filed charges against Garber alleging violations of federal safe drinking water laws, conspiracy, falsifying documents and statements, and cover-up.

Purdon said the Environmental Protection Agency, the U.S. Justice Department and the U.S. attorney’s office have an ongoing federal criminal investigation into Halek Operating, a company that is now dissolved. Purdon said no charges have been filed in that investigation. He declined to comment further.

State officials say the Stark County site has not been cleaned up because it is being used as evidence in the federal investigation. Officials said it doesn’t pose a threat to drinking supplies, and that the entire $1.5 million state fine may never be collected from Halek.

The state has recovered $140,000 of the fine against Halek Operating to date, including the seizure of a $100,000 bond this week that had been held at North Dakota’s state-owned bank, said Alison Ritter, a North Dakota Department of Mineral Resources spokeswoman.

“We know of no other assets at this time,” Ritter said.