-- Secretary Steven Mnuchin requested use of a government jet to take him and his wife on their honeymoon in Scotland, France and Italy earlier this summer, sparking an inquiry by the Treasury Department's Office of Inspector General, sources tell ABC News.
Officials familiar with the matter said the highly unusual ask for a U.S. Air Force jet, which according to an Air Force spokesman could cost roughly $25,000 per hour to operate, was put in writing by the secretary's office but was deemed unnecessary after further consideration by Treasury Department officials.
Sen. Ron Wyden (D-Ore.), the top Democrat on the Senate Finance Committee, said in an interview with ABC News that Mnuchin's request for a government jet on his honeymoon defies common sense.
"You don't need a giant rulebook of government requirements to just say yourself, 'This is common sense. It's wrong,'" Wyden said. "That's just slap-your-forehead stuff."
Mnuchin, an independently wealthy former Goldman Sachs banker, has already triggered a review of his travel for using a government jet to travel to Louisville and Fort Knox, Kentucky last month. The inspector general is reviewing whether he improperly used that trip to catch a prime view of the solar eclipse with his wife, Louise Linton, a Scottish actress and model.
Senate Majority Leader Mitch McConnell of Kentucky met with Mnuchin during that trip and tweeted a photo of them watching the eclipse together, complete with proper eyewear.
The secretary's office denied he took that trip to watch the eclipse and said he was there to attend meetings on a tax code overhaul, and the Treasury Department said the Mnuchins would reimburse the government for Linton's travel costs.
An official in the Treasury Department's Office of Inspector General said that in addition to reviewing the Kentucky trip, it has started an official inquiry into Mnuchin's honeymoon travel request.
A spokesman for the Treasury Department told ABC News that Mnuchin requested government travel for his honeymoon out of a concern for maintaining a secure method of communication.
"The secretary is a member of the National Security Council and has responsibility for the Office of Terrorism and Financial Intelligence," the spokesman said in a statement. "It is imperative that he have access to secure communications, and it is our practice to consider a wide range of options to ensure he has these capabilities during his travel, including the possible use of military aircraft."
The spokesman added that the secretary's office decided the use of military aircraft was "unnecessary" after it became apparent that other methods for secure communication were available.
Aside from the president and vice president, travel on military aircraft is typically reserved for Cabinet members who deal directly with national security, such as the secretaries of defense and state.
One senior Treasury official who has worked with a number of past secretaries said that military aircraft are used only in "extreme" circumstances — for example, if the secretary has to be rushed back to a meeting in Washington, D.C., with the president.
Another former senior Treasury official, who worked closely with Mnuchin's predecessor Jack Lew, said it would have been "exceedingly rare" for Lew to use military aircraft for official business. The only exception was for foreign business travel. As for private travel, "there's not a chance in hell that Secretary Lew would have considered using military air," the former official said.
Adam Stump, a spokesman with the Department of Defense, which oversees and operates all government air travel for the executive branch, declined to comment on the specific request made by Mnuchin's office but cited existing departmental policies regarding the use of government aircraft.
"Generally, when other federal executive agencies request use of military airlift, it is provided on a reimbursable basis pursuant to Title 31 U.S.C., section 1535 and 1536, otherwise known as the Economy Act," Stump said.
Noah Bookbinder, the executive director of Citizens for Responsibility and Ethics in Washington, a D.C.-based ethics watchdog, was critical of Mnuchin's request.
"People can do whatever they want on their own time, on their vacations and in the houses that they live in, but they can't be expecting taxpayers to foot the bill for a Hollywood lifestyle," Bookbinder said.
Meanwhile, Linton managed to stir her own controversy surrounding the August trip to Kentucky when she lashed out at a stranger on Instagram — for which she later issued a public apology. She posted a photo of herself and Mnuchin stepping off a government jet and wrote, "Great #daytrip to #Kentucky! #nicest #people #beautiful #countryside."
She went on to include hashtags of various luxury designers she was wearing: "#rolandmouret pants #tomford sunnies, #hermesscarf #valentinorockstudheels #valentino #usa," prompting one user to reply, "Glad we could pay for your little getaway. #deplorable."
Linton responded by belittling the woman in a series of comments and even mentioned her honeymoon.
"Aw!! Did you think this was a personal trip?! Adorable! Did you think the US govt paid for our honeymoon or personal travel?! Lololol."
Two people familiar with the matter say Linton was not aware that her husband had requested government travel for their honeymoon before making that comment.
ABC News' Ben Siegel, Matthew Mosk, Luis Martinez and Devin Dwyer contributed to this report.