Government investigators say the former US interior secretary Ryan Zinke misused his position to advance a development project in his Montana home town and lied to an agency ethics official about his involvement.

The interior department’s inspector general said in a report made public on Wednesday that Zinke continued working with a foundation on the commercial project in the community of Whitefish, Montana, even after he committed upon taking office to breaking ties with the foundation.

The report also says Zinke gave incorrect and incomplete information to an interior department ethics official who confronted him over his involvement and that Zinke directed his staff to assist him with the project in a misuse of his position.

The Great Northern Veterans Peace Park Foundation was established by Zinke and others in 2007. Zinke and his wife were in negotiations with private developers for the use of foundation land for a commercial development project.

Zinke is a candidate in the June Republican primary for an open Montana congressional seat, a position he held before joining Trump’s cabinet.

He stepped down from his role as interior secretary in the Trump administration in December 2018 following a series of scandals in which he was accused of using his position for personal gain.

On Wednesday Zinke’s campaign called the report “a political hit job” and said his family’s involvement in the land deal led to the creation of a children’s sledding park.

Investigators referred the matter to the Department of Justice for potential prosecution but they declined to pursue a criminal case, according to the report.

The investigation into the land deal was one of several focused on Zinke that began when he was in Trump’s cabinet.

In another case, investigators found that he violated a policy that prohibits non-government employees from riding in government cars after his wife traveled with him, but he said ethics officials approved it.

Zinke was cleared of wrongdoing following a complaint that he redrew the boundaries of a national monument in Utah to benefit a state lawmaker and political ally.

Under Zinke’s leadership, the interior department sought to advance oil and gas drilling and mining on or near public land, rolled back protections for threatened species and shrank national monuments.

“Zinke’s days of plundering our lands and enriching himself and his friends are over,” said Nicole Ghio, senior fossil fuels program manager for Friends of the Earth, said at the time he stepped down. Trump had praised him saying he had “accomplished much during his tenure”.

Associated Press contributed to this report