Secretary of the Treasury Steven Mnuchin and Ivanka Trump arrive with President Trump at Bismarck Municipal Airport in Bismarck, N.D., on September 6, 2017. (Brendan Smialowski/AFP/Getty Images)

Treasury Secretary Steven Mnuchin flew on a government jet from New York City to Washington following a news conference with President Trump last month at Trump Tower, according to a Treasury investigator now reviewing the trip.

The flight, first reported by ABC, joins a series of expensive taxpayer-funded trips taken by Trump's Cabinet secretaries, which critics have slammed as a waste of public funds.

Mnuchin flew to Washington aboard a C-37 piloted and maintained by the U.S. Air Force after Trump's Aug. 15 news conference, during which the president blamed "both sides" for brutal violence days earlier during a clash of neo-Nazis and protesters in Charlottesville, Va.

Mnuchin, a former banker and hedge-fund manager who was joined at the brief event by Transportation Secretary Elaine Chao, stood behind Trump and offered no comments during the event. The news conference was called to discuss the administration's infrastructure plans.

Federal agencies who request government jets are required to reimburse the military for travel expenses. The C-37, a military-outfitted version of the Gulfstream V executive jet, has a reimbursable rate of up to $10,000 an hour, Pentagon documents show.

The 200-mile path between New York and Washington is one of the most well-connected travel routes in the country, and millions have maneuvered it via one-hour commercial flights and three-hour trips aboard passenger trains such as Amtrak's Acela Express.

Defense Department policy calls government air transportation "a premium mode of travel involving high costs and limited resources" and says "every effort shall be made to minimize travel cost."

Officials told ABC that Chao also took the government jet for at least one leg of the Trump Tower appearance. Agency officials did not respond to requests for comment.

A Department of Defense spokesperson referred questions to the Air Force, which did not immediately respond.

The Trump Tower flight was less than a week before Mnuchin and his wife, Louise Linton, flew to Louisville, Ky., on a government jet to attend a luncheon and visit the nation's gold vault at Fort Knox, where Mnuchin also viewed the solar eclipse.

Mnuchin's office also requested a government jet fly him and Linton on a honeymoon trip to Europe this summer. Treasury officials said the request was made to guarantee access to secure communications during the trip and was withdrawn before any flight.

An official in the Treasury Department's Office of Inspector General said both Mnuchin's Trump Tower and Fort Knox flights are currently under review.

The official declined to offer further details until officials are "able to perform a complete analysis of how these trip requests and authorizations were staffed and reviewed."

In a statement, a Treasury spokesperson told The Washington Post, "We welcome the OIG's review and are ensuring the office has everything needed for a full evaluation of our travel procedures."

Sen. Ron Wyden (D-Ore.) last month requested a "detailed explanation" of how Mnuchin justified taking the military plane to Fort Knox. A Treasury official wrote back this week saying the agency "considered a number of travel options" before requesting the government jet.

The Mnuchin case marks at least the third open review by an inspector general into travel expenses for Cabinet secretaries, who have traditionally flown on cheaper commercial airlines for domestic flights.

The Health and Human Services inspector general is investigating reports that Secretary Tom Price flew aboard expensive chartered planes for at least two dozen taxpayer-funded flights, a spokeswoman said Friday. An official in Price's office defended the flights as "Secretary Price, getting outside of D.C., making sure he is connected with the real American people."

The Environmental Protection Agency's inspector general also announced last month that it had begun investigating Administrator Scott Pruitt's frequent travel to his home state of Oklahoma.

Rep. Elijah E. Cummings (D-Md.), the ranking member of the House Oversight Committee, said in a statement Friday that "too many Trump Administration officials have an entitled, millionaire mindset when it comes to squandering taxpayer money."