The Republican chairman on the House Natural Resources Committee has joined Democratic lawmakers who have taken an interest in the travel expenses incurred by Interior Secretary Ryan Zinke.

In a letter sent late Tuesday, Rep. Rob Bishop, R-Utah, whose committee has oversight of Interior, asked the agency chief to provide details about its travel policies and travel records for Interior secretaries over the last eight years. The letter, which was also signed by the panel’s Oversight and Investigations Subcommittee Chairman Bruce Westerman, R-Ark., also suggested that Bishop believes Democrats may be trying to draw attention to the issue for political purposes.

“When willful violations occur, there should be consequences,” Bishop and Westerman wrote in the letter. “When partisan opportunists conflate diligent conformance to scandal, no one wins. Let’s get all facts on the table, ensure taxpayers are protected and proceed with the peoples’ business.”

Zinke is so far the fourth Trump administration cabinet official under scrutiny for chartering private or military aircraft for work-related travel instead of using cheaper commercial airlines. Health and Human Services Secretary Tom Price resigned on Friday over his travel expenses. EPA Administrator Scott Pruitt and Treasury Secretary Steve Mnuchin are under a microscope for similar travel practices.

The Interior Office of Inspector General opened an investigation into the issue on Friday as news reports detailed flights Zinke and his wife Lolita had taken since he took office.

Those flights included a $12,000 charter plane to Zinke’s hometown in Montana and private flights between St. Croix to St. Thomas in the U.S. Virgin Islands. Zinke and his wife in May also reportedly used a military aircraft to fly to Norway and then to Alaska for events organized by the Senate Energy and Natural Resources Committee.

After a separate complaint from the Campaign for Accountability , the U.S. Office of Special Counsel has also agreed to look into whether in his travels, Zinke violated the Hatch Act by giving a speech at a major political donor’s event in Las Vegas in June. The statute prohibits cabinet members from engaging in political activity.

Zinke has dismissed the reports his travel was improper, and said he received approval for his plans from Interior’s ethics officials before taking the flights. His office was not immediately reachable for comment.

Bishop’s letter asked for documents related to Interior’s policies and guidelines for secretary travel when using government-owned, rented, leased, or chartered aircraft and “each use of a government-owned, rented, leased, or chartered aircraft by the secretary since January 20, 2009.”

That date was when former President Barack Obama was inaugurated for his first term in office.

“Ethical guidelines are on the books to promote transparency and responsible use of taxpayer dollars. Federal officials should be held to the highest ethical standard in adhering to these rules,” the Republicans’ letter said. “When violations occur, the public deserves to know.”

But the letter also appears to defend Zinke from a formal request to the Interior Office of Inspector General for an investigation by Natural Resources top Democrat Raúl  M. Grijalva of Arizona

The Republican lawmakers said that Grijalva in his request to the OIG had “selectively” left out a statement from an agency spokeswoman that the private and military planes were used only after officials had failed to find commercial flights that would meet Zinke’s schedule and that they had been pre-cleared by Interior’s ethics office.

Zinke’s office has until Oct. 17 to deliver the requested documents.