Interior Secretary Ryan Zinke’s travel practices are under investigation by the agency’s inspector general. (Zach Gibson/Bloomberg)

Interior Secretary Ryan Zinke has failed to keep complete records — and in some cases, kept none at all — of his travel since taking office, the agency’s watchdog told department officials this week, saying that management of Zinke’s travel was “deficient” and lacked oversight.

A rare alert Deputy Inspector General Mary Kendall sent to the secretary's office Wednesday, obtained by The Washington Post, said her investigation into allegations of improper travel practices by Zinke has been stymied by "absent or incomplete documentation for several pertinent trips."

Interior lawyers and ethics officials also have not shown evidence to investigators that they have been able to “distinguish between personal, political and official travel” or cost-analysis documents to justify his choice of military or charter flights, Kendall wrote.

The memo reveals that the inspector general is also scrutinizing the travel of Zinke’s wife, Lolita, who often accompanied him on official trips. Kendall wrote that the department’s documentation was so lacking that investigators cannot determine “the full extent” of her travel and how it was paid for.

The alert raises new questions about the travel of the former Navy SEAL commander and former congressman from Montana. The Post and other news organizations have reported that Zinke flew on a private plane owned by an oil executive, took charter flights in the Virgin Islands, and has mixed official travel with stops at political fundraisers and donor events.

Interior Department officials have said that all of Zinke’s travel was approved in advance by ethics officials and that private-charter flights were booked only when feasible commercial flights were unavailable.

But the agency declined to publicly release documents authorizing his travel or to say what percentage, if any, of the cost of trips that included political appearances was covered by Republican campaign funds, as would be required.

Kendall’s office has requested complete travel records for Zinke and his wife, including authorizations and reimbursement documents that would “describe all instances when Mrs. Zinke traveled in a government-owned vehicle, watercraft, or aircraft,” including the details of who paid.

In a letter of response obtained by The Post, Deputy Secretary David Bernhardt told Kendall that the department would provide the outstanding travel documents. But he blamed the Obama administration for any record-keeping issues.

“When I arrived at the Department in August 2017, it was clear to me that the Secretary and I inherited an organizational and operational mess from the previous administration,” Bernhardt wrote. “From my perspective . . . it appears that the exact same [travel] procedures and processes utilized by the previous Administration remain in place and continue to be dysfunctional.”

Bernhardt added that it was “apparently unclear” until last week whether all of former Interior Secretary Sally Jewell’s travel records had been entered into the agency’s tracking system.

In recent weeks, Democratic lawmakers have called for an investigation into Zinke’s travel. “I urge you to provide a full accounting of the facts and a determination of consistency with the law,” Rep. Raúl M. Grijalva (D-Ariz.) and Rep. A. Donald McEachin (D-Va.) wrote in a letter last month.

The Office of Special Counsel, an independent agency, also is investigating Zinke’s travel to political fundraisers and other appearances for possible violations of the Hatch Act, which bans federal officials from engaging in political activity on the job.

Zinke is one of four current and former Trump administration Cabinet secretaries under investigation by agency watchdogs. Tom Price, another former member of Congress who was secretary of health and human services, resigned in late September after taking at least $400,000 in chartered flights at taxpayer expense.

The watchdog for the Environmental Protection Agency also is investigating Administrator Scott Pruitt’s frequent travels to his home state of Oklahoma. Veterans Affairs Secretary David Shulkin’s July trip to Europe — which combined official travel for himself, his wife and two top aides with sightseeing and a Wimbledon tennis event — also is under review.

The Treasury Department inspector general’s office found in October that Secretary Steven Mnuchin’s multiple flights on military jets to destinations from West Virginia to Italy were legal but that Treasury officials had offered poor justifications for the travel.