“We welcome the [Office of the Inspector General's] review and are ensuring the office has everything needed for a full evaluation of our travel procedures," the Treasury Department said in a statement.
Chao took the C-37 jet from Joint Base Andrews to Teterboro Airport in New Jersey for the Aug. 15 press conference, and Mnuchin, who flew up commercially, used it to return to D.C., according to their department spokespeople.
When asked for an explanation about who ordered the government jet for travel between New York and Washington, a Department of Transportation spokesman insisted it did not come from his department. A spokesman for the Treasury Department declined to comment.
However, two Defense Department officials told ABC News that U.S. Air Force records show Mnuchin's office requested the flight and that Chao was later added to it. According to the Defense Department, it costs $25,000 per hour to operate the C-37, the military's equivalent of a Gulfstream jet.
A spokesman for the Department of Defense also told ABC News that "generally, when other federal executive agencies request use of military airlift, it is provided on a reimbursable basis." That reimbursement, however, generally matches an equivalent coach fare, rather than the total cost to operate the aircraft.
Chao's office said she only takes government travel if there are concerns about security, excessive cost, or if there are no commercial options available. Yet her spokesman could not say which of these criteria she used to justify her government flight on Aug. 15. Multiple carriers shuttle hourly flights between Washington and New York that can be booked on the same day for less than $1,000.
Mnuchin is not the only member of Trump's cabinet whose travel has come under scrutiny.
ABC News has confirmed that Health and Human Services Secretary Tom Price is now under investigation by his department's inspector general for chartering dozens of private flights for domestic business trips. Politico reported this week the bills for those flights, footed by American taxpayers, amount to roughly $300,000.
A spokesman for Price told ABC News the private flights were necessary, in part, because of his demanding schedule.
"The Secretary has taken commercial flights for official business after his confirmation. He has used charter aircraft for official business in order to accommodate his demanding schedule. The week of September 13 was one of those times, as the secretary was directing the recovery effort for Irma, which had just devastated Florida, while simultaneously directing the ongoing recovery for Hurricane Harvey," the spokesman said.
But Price also took a flight in June from Washington to Philadelphia at the same time commercial carriers were flying that route. The price of a 40-minute commercial flight from Washington to Philadelphia typically falls in the hundreds of dollars range. Private charter companies typically charge a two-hour, $10,000 minimum for the day.
Today, the ranking member of the House Oversight Committee, Rep. Elijah Cummings (D-Md.), called on Republicans to hold hearings on the administration's use of costly travel.
“Too many Trump Administration officials have an entitled, millionaire mindset when it comes to squandering taxpayer money that does not belong to them just to support their lavish lifestyles,” Cummings said in a statement. “This starts at the very top, and the American people are not going to keep footing the bill for the Trump Administration’s champagne wishes and caviar dreams."
In addition, investigators at the Environmental Protection Agency's are examining Administrator Scott Pruitt's "frequent travel to and from his home state of Oklahoma at taxpayer expense," to determine if they were done in accordance with EPA policy, according to a memo from the department's office of inspector general.
ABC News' Erin Dooley, Jordyn Phelps and Katherine Faulders contributed to this report.