BILLINGS, Mont. (AP) – Three of seven defendants charged in a corruption case on Montana’s Crow Indian Reservation have pleaded guilty to federal theft charges after they were accused of taking money meant to pay for monitoring historic and cultural sites, including a bison kill site that was later severely damaged.

Troy Chandler, Mark James Denny and Frederick Deputee, Jr. pleaded guilty in U.S. District Court Wednesday, following plea deals with prosecutors earlier in the week under which additional charges would be dropped.

Under the agreement, Denny and Chandler would pay restitution, and each of the three defendants could face up to 10 years in prison, a $250,000 fine and three years’ supervised release.

The men did work for the Crow Tribal Historic Preservation Office, which monitored companies including a coal mine on the reservation along the Wyoming border.

Under former preservation office director Dale Old Horn, authorities said companies were told to pay monitors directly instead of paying the Crow Tribe as should have been required. Some of the monitors were paid a second time as tribal employees.

From July 2009 to November 2011, companies paid more than $500,000 for monitoring services by the defendants, authorities said. Among the sites monitored by Denny and Deputee was a 2,000-year-old bison kill site that the U.S. Attorney’s Office said was irreparably damaged when excavation work was approved in 2011 by Old Horn.

Charges in the case followed a multi-agency investigation conducted as part of the U.S. Attorney’s Guardians Project, which aims to purge corruption lined to the administration of federal programs on Montana’s Indian reservations.

Chandler and Denny, who were tribal employees, pleaded guilty during Wednesday court appearances to theft from an organization receiving federal funding. The Historic Preservation Office receives money from the National Park Service to ensure that lands of cultural or historic importance are not destroyed.

Assistant U.S. Attorney Carl Rostad said in court filings that the government will seek $44,546 in restitution from Chandler and $73,046 from Denny. The Billings Gazette reported that Denny plans to challenge the government’s calculations. The amount involved could affect sentencing.

Deputee, Old Horn’s grandson, pleaded guilty to theft from an Indian organization. He was not an employee of the preservation office but was assigned to monitor a planned expansion of Westmoreland Resource’s Absaloka coal mine near the location where the bison kill site was excavated.

He was accused stealing $15,325 from GCM Services, a subcontractor hired by Westmoreland to do archaeological work.

Sentencing for the defendants pleading guilty was set for May 1 before Judge Richard Cebull. The three men are currently released on special conditions set by the court.

Old Horn and three others charged in the case are awaiting a June 10 jury trial on charges including fraud, conspiracy and extortion.

Cebull this week cancelled a February trial date after attorneys for the defendants requested more time to prepare.