As the Interior Department inspector general has carried out its investigation into Interior Secretary Ryan Zinke’s travel habits, the watchdog has discovered that the department has not properly documented Zinke’s travel, according to a letter obtained by the Washington Post.

In a letter to Interior officials, Deputy Inspector General Mary Kendall wrote that the probe has been delayed due to “absent or incomplete” records. She said that the department had yet to finalize vouchers and authorizations for Zinke’s travel, and that Interior officials have not provided sufficient documentation of the legal and ethical analysis required to distinguish between personal and official travel.

Zinke is one of several cabinet leaders in the Trump administration who has come under scrutiny for his travel on non-commercial planes. The inspector general is looking at Zinke’s reported use of a private plane to attend meeting with the Vegas Golden Knights hockey team, which is owned by a major donor to Zinke’s 2014 congressional campaign.

Zinke also used non-commercial planes on several occasions, including for trips between St. Thomas and St. Croix, Politico reported in September.

The inspector general’s office is also looking at the travel habits of Zinke’s wife, Lolita Zinke, according to the Wednesday letter. Kendall wrote that her office has been unable to determine how many trips Lolita Zinke went on with her husband and who paid for her travel.