The Interior Department's internal watchdog has opened an investigation into a real estate deal involving Interior Secretary Ryan Zinke and the head of the Hallburton energy company.

The investigation was first reported by Politico and confirmed to Fox News by Nancy DiPaolo, a spokeswoman for the inspector general's office.

The investigation centers on a foundation Zinke established to build a park in his home state of Montana. In September of last year, Zinke's wife, Lola, signed an agreement allowing a group of developers -- including Halliburton Chairman David Lesar -- to use its land to build a parking lot for a development in Zinke's hometown.

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.

Last month, Zinke affirmed he had met with Lesar and his son at the Interior Department in August of last year. However, he insisted that he provided them only with background on the nonprofit and the land it owned.

Democrats had called for Zinke to be investigated in order to determine whether the deal violated conflict-of-interest rules.

"Secretary Zinke doesn’t seem to take his responsibility to the public seriously," Rep. Raul Grijalva, D-Ariz., told Politico on Wednesday. "He's turned it into the Ryan Zinke show, which is more about waving his own flag above the building and doing personal business deals with his friends instead of protecting public lands and improving our environmental quality. This formal investigation is one of many he’s managed to pile up in his short and undistinguished tenure, and I join my Democratic colleagues in seeking the transparency and accountability that Republicans have so far not provided."

Zinke, a former Montana congressman, has not commented to Fox News on the opening of the investigation.

Fox News' Kelly Chernenkoff and The Associated Press contributed to this report.