WASHINGTON (Reuters) - The U.S. Interior Department has failed to provide documentation needed for an internal probe of Secretary Ryan Zinke’s travels, the department’s inspector general said on Thursday.

U.S. Interior Secretary Ryan Zinke (C) talks to National Park Service Rangers, while traveling for his National Monuments Review process, in Boston, Massachusetts, U.S., June 16, 2017. Picture taken June 16, 2017. REUTERS/Brian Snyder

“Our investigation has been delayed by absent, or incomplete documentation for several pertinent trips,” Mary Kendall, the department’s deputy inspector general, said in a letter sent this week to David Bernhardt, the department’s deputy secretary. The letter was published on the department’s web site.

The inspector general launched an investigation of Zinke’s travels after reports emerged in September that he had used a private plane owned by oil executives. The probe was launched after the inspector general received numerous complaints about Zinke’s travels, including the use of three chartered flights.

One of those flights, taken in June from Las Vegas to near Zinke’s hometown in Montana, cost taxpayers over $12,000, according to a Washington Post report.

Kendall’s letter said her office had received full cooperation from all employees it had contacted, but that “we have found the documentation and adherence to departmental travel policies deficient and without proper management oversight and accountability.”

It said many authorizations and vouchers required for Zinke’s travel have yet to be completed and processed. It requested documentation no later than Dec. 11 for the secretary and his wife, Lolita Zinke, when she accompanied him on government travel.

The letter also complained that the department’s travel review process failed to include proper documentation and accountability.

The Interior Department had no comment, but referred to a letter, also published on the web site, from Bernhardt responding to Kendall.

In a speech at conservative think tank the Heritage Foundation in September, Zinke denied inappropriate travel and said his travels were approved by “career employees” of the Interior Department’s ethics office. He called reports about his use of chartered private flights “a little B.S.”

Bernhardt said in his letter that additional documents for the probe had been sent to the inspector general’s office earlier in the month. He said the department will work to provide available documentation for Zinke’s travel and said he appreciated recommendations by Kendall on travel procedures for the secretary’s immediate office.

Bernhardt also said that Zinke and he had “inherited an organizational and operational mess” from the Obama administration. He said department travel procedures were the same under Obama.

EPA Administrator Scott Pruitt and Treasury Secretary Steve Mnuchin have also come under scrutiny over reports of expensive private plane use.