DENVER (AP) — The U.S. Interior Department’s internal watchdog has opened an 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 a company co-owned by Halliburton chairman David Lesar to use land for a commercial development in Zinke’s hometown in Montana.

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.