In the aftermath of Secretary of Health and Human Services Tom Price's resignation from his position last week, several administration officials continue to be scrutinized for their travel plans.

Price, who was reportedly asked to resign by President Donald Trump, repeatedly utilized private aircraft at taxpayer expense for domestic travel, including on trips to destinations as close to Washington, D.C. as Philadelphia.

Trump expressed his dissatisfaction with Price on multiple occasions, but has remained largely silent on the other members of his administration about whom questions have been raised.

At least four other officials have been identified for their use of chartered jets or military aircraft as opposed to taking commercial flights. In response, the House Oversight Committee initiated an inquiry last week into federal agency travel. The committee has asked the White House and all 24 federal agencies to account for all senior officials' travel since the inauguration by Oct. 10.

Here's a look at the controversies:

Treasury Secretary Steve Mnuchin

Treasury Secretary Steven Mnuchin holds a news conference, Sept. 21, 2017 in New York City.  (Spencer Platt/Getty Images) Treasury Secretary Steven Mnuchin holds a news conference, Sept. 21, 2017 in New York City.

On Sept. 13, ABC News was the first to report that Mnuchin requested the use of government plane to fly him and his wife, actress Louise Linton, to Europe for their honeymoon over the summer. The request led to an inquiry by the Treasury Department's Office of the Inspector General.

He later defended the action, claiming that a government jet would have provided him with a secure line of communication to the United States and therefore the request was in the interest of "national security." Officials familiar with the request characterized it as unusual, and it was ultimately withdrawn.

The Air Force says it costs roughly $25,000 an hour to operate its C-37, the military's equivalent to a Gulfstream jet. It charges outside agencies roughly 10,000 an hour to fly in one.

This is the second time Mnuchin has drawn the scrutiny of his department's inspector general. When the news emerged about the Europe trip, he was already the subject of an inquiry, after he and Linton traveled on a government jet to Kentucky on Aug. 21, when the total solar eclipse was visible there.

A Treasury Department spokesperson at the time portrayed the timing of the trip and the eclipse as coincidental, saying "the eclipse did not even factor in the travel decision."

The trip received additional publicity due to an Instagram post by Linton that showed her and Mnuchin walking down the stairs of the plane and included hashtags of the designer brands she was wearing. After being criticized in a comment on the post, Linton responded, writing, "Aw!! Did you think this was a personal trip?! Adorable! Did you think the US govt paid for our honeymoon or personal travel?! Lololol."

HHS Secretary Tom Price

Health and Human Services Secretary Tom Price speaks at the Mirmont Treatment Center in Media, Pa., Sept. 15, 2017. (Jacqueline Larma/AP, File) Health and Human Services Secretary Tom Price speaks at the Mirmont Treatment Center in Media, Pa., Sept. 15, 2017.

Price's trips aboard chartered private jets were revealed by Politico last week in a story that reported he took five flights aboard such planes in the preceding week, eschewing commercial flights or travel by train or car, which could have saved tens of thousands of dollars.

A spokesperson for Price said that "every possible source for travel needs" is checked "but commercial travel is not always feasible."

Politico later reported that the number of private flights taken by Price this year was as high as 26 and that he took military planes to Africa and Europe where he was accompanied by his wife. The total estimated cost of all of Price's private and military travel has eclipsed $1 million, according to Politico.

In the wake of the news, the Democratic ranking members of five congressional committees called on the Health and Human Services Office of Inspector General last week to investigate the matter. A spokesperson for the inspector general said in September that an investigation was underway, that the office was taking the matter "very seriously" and that the scope of the inquiry could expand, if necessary.

Trump's comments last week were the first public indication since the news broke that he did not have full confidence in Price.

"I will tell you, personally, I'm not happy about it," said the president.

At the White House Thursday, press secretary Sarah Sanders explained that the White House is not involved in approving the use of private planes by federal agencies, but that it has asked HHS to put "a halt" on chartered flights until the reviews of the department's travel are completed.

Price expressed "regret" shortly afterward and pledged to reimburse the government for his portion of the 26 private flights, nearly $52,000. He resigned shortly on Friday to allow Trump and the administration to "move forward without further disruption," he said in a statement.

It is not yet clear whether Price has followed through on his promise to reimburse the Treasury for his flights.

EPA Administrator Scott Pruitt

Environmental Protection Agency (EPA) Administrator Scott Pruitt speaks in Washington, Feb. 21, 2017.(Susan Walsh/AP) Environmental Protection Agency (EPA) Administrator Scott Pruitt speaks in Washington, Feb. 21, 2017.

Last week, the Environmental Protection Agency confirmed that Pruitt flew aboard one charter flight and three government flights. The EPA is one of the agencies that was asked to provide information about senior officials' travel to Congress.

Pruitt's travel costs totaled more than $58,000 and included a military flight to Ohio, an interagency charter in Oklahoma, a private flight in Colorado and a trip aboard a plane owned by North Dakota.

An EPA public affairs officer tweeted that, aside from those four flights, Pruitt "flies commercial."

The EPA's Office of the Inspector General initiated an audit of his travel this year after travel expense documents were released showing frequent trips to Oklahoma, where he previously lived and served as state attorney general. The audit is ongoing.

Pruitt is also under scrutiny for installing a $25,000 private phone booth in his office, which he said he needs to make secure phone calls.

Interior Secretary Ryan Zinke

Interior Secretary Ryan Zinke addresses criticism of his travel practices before delivering a speech billed as "A Vision for American Energy Dominance," at the Heritage Foundation, Sept. 29, 2017, in Washington.(Mark Wilson/Getty Images) Interior Secretary Ryan Zinke addresses criticism of his travel practices before delivering a speech billed as "A Vision for American Energy Dominance," at the Heritage Foundation, Sept. 29, 2017, in Washington.

Zinke chartered three flights since March at a cost of $12,375, an Interior Department spokesperson confirmed Thursday, including between two of the U.S. Virgin Islands and to Montana from Las Vegas in June.

“The secretary traveled on charter flights when there were no commercial options available,” explained spokesperson Heather Swift, who added that the requests had been reviewed by the department's ethics office.

Zinke further traveled on a military plane with Agriculture Secretary Sonny Perdue to visit a wildfire camp in Montana and meet with firefighters. A USDA spokesperson said there were "no viable commercial options available" for the trip, which included Zinke, Perdue and staff and security from multiple agencies.

Counselor to the President Kellyanne Conway

Kellyanne Conway, Counselor to President Donald Trump, poses for a photograph during an interview at the White House, Aug. 3, 2017 in Washington, D.C. (Chris Kleponis-Pool/Getty Images) Kellyanne Conway, Counselor to President Donald Trump, poses for a photograph during an interview at the White House, Aug. 3, 2017 in Washington, D.C.

Following Price's resignation, the focus of at least one member of Congress turned to Conway after the revelation that the counselor to Trump accompanied the health secretary on a number of his flights.

Rep. Elijah Cummings, D-Md., the ranking member on the House Oversight Committee, authored a letter to Conway to ask for additional information about her travel on non-commercial planes.

"Despite the fact that you joined Secretary Price on several of these flights, you have not made any similar public statements indicating whether your own actions were appropriate, whether you will continue to take such flights at taxpayer expense in the future, or whether you plan to personally repay the taxpayers for the cost of your seats on these flights,” said Cummings in the letter, obtained by ABC News.

In response to questions about Conway's travel, a White House official seemed to shift the culpability to HHS.

"Agencies are responsible for arranging appropriate transportation for their own events… When White House staff are invited, their travel plans are planned and secured by the inviting agency," said the official.

ABC News' Stephanie Ebbs, Katherine Faulders, Justin Fishel, Alexander Mallin and Benjamin Siegel contributed to this report.