The U.S. Interior Department’s internal watchdog has opened a formal investigation into Secretary Ryan Zinke’s involvement in a land deal with the head of an energy services company that does business with the agency.

Deputy Inspector General Mary Kendall said in a letter Wednesday to congressional Democrats that the investigation started Monday.

The Associated Press reported last month that Kendall’s office was looking into lawmakers’ complaints over a charitable foundation created by Zinke and run by his wife, Lola.

The foundation allowed 95 Karrow LLC, a company co-owned by Halliburton chairman David Lesar, to use land for a commercial development in Zinke’s hometown of Whitefish.

Zinke spokeswoman Heather Swift has said he did nothing wrong and that Zinke resigned from the foundation’s board of directors prior to the land deal.

In June, inspector general spokeswoman Nancy DiPaolo confirmed that the inspector general had opened an “investigative complaint into purported business activities by the secretary” and that “a preliminary review is underway.”

The Western Values Project, a public-lands advocacy group based in Whitefish, welcomed the investigation.

“This entire episode is a perfect metaphor for Secretary Zinke’s career,” said Executive Director Chris Saeger. “He’s been using public lands to serve himself, and the rest of us have paid the price. We applaud this important step toward finally holding him accountable.” Western Values is a frequent critic of Zinke and President Trump.

The Daily Inter Lake has reported on the proposed land deal as it worked its way through the Whitefish planning process before winning City Council approval in late 2017.

The crux of the alleged conflict of interest is a shared-use agreement between 95 Karrow and the Great Northern Veterans Peace Park Foundation for a parking lot to be built on the private park land.

The Great Northern Veterans Peace Park located next to the 95 Karrow development site was a proposal pushed by Zinke about 10 years ago. BNSF Railway donated the land for what Zinke envisioned as a public sledding hill, to Zinke’s Great Northern Veterans Peace Park Foundation, but the park property remains undeveloped open space.

Zinke is no longer affiliated with the Veterans Peace Park Foundation. According to Whitefish attorney Sean Frampton, legal representative for the Great Northern Veterans Peace Park Foundation, Zinke resigned as an officer and director in 2017.