Four members of President Trump's Cabinet have become embroiled in a growing controversy over the use of private or military aircraft, costing thousands of dollars at the taxpayers' expense, and more names could soon be added to that list.

Health and Human Services Secretary Tom Price turned in his letter of resignation Friday after facing fierce backlash over his repeated use of private and military aircraft to get around.

While Price may have been the leading figure in the whole controversy, he was far from alone.

Here's a look at what we know about the situation surrounding Price and the remaining Trump administration officials facing scrutiny over their travel habits — Treasury Secretary Steven Mnuchin, EPA Administrator Scott Pruitt, and Interior Secretary Ryan Zinke — and who might be next.

  • Treasury Secretary Steven Mnuchin

The whole private jet debacle began just over a month ago, at the end of August, with Mnuchin, a former hedge fund manager, and his new wife, Louise Linton. Linton grabbed headlines after she posted a photo on Instagram bragging about her designer outfit and accessories. She subsequently clashed with a women on social media and insinuated she had sacrificed more for the U.S. because she was rich. But it was the photo that she posted that caught the attention of a watchdog group because it showed her and Mnuchin departing a government plane during a trip to Kentucky. Citizens for Responsibility and Ethics in Washington filed a Freedom of Information Act request for copies of records concerning authorization for, and the costs of, Mnuchin's use of a government plane to travel to Lexington, Ky.

Mnuchin's trip took place on Aug. 21, the same day as the total solar eclipse, and included a luncheon at the Louisville Chamber of Commerce with Senate Majority Leader Mitch McConnell and a visit to the gold bullion depository in Fort Knox, just south of Louisville. McConnell later posted a picture on Facebook saying he and Mnuchin watched the eclipse from the rooftop

CREW said the requested records would "shed light on the justification for Secretary Mnuchin's use of a government plane, rather than a commercial flight, for a trip that seems to have been planned around the solar eclipse and to enable the Secretary to secure a viewpoint in the path of the eclipse's totality."

Treasury Department officials have defended Mnuchin's viewing of the eclipse, saying that it was planned around "official government travel."

Mnuchin also raised eyebrows when reports revealed he inquired about using a military plane for his honeymoon in Europe over the summer -- a move he justified as being related to national security. He withdrew the request when he found another option. He also flew on a military jet back to Washington, D.C., following an appearance at Trump Tower in New York City last month. Mnuchin said he had a call lined up that needed to be secure. "There are times when I need secure communications to be in touch with the president and the National Security Council," Mnuchin said on ABC's "This Week."

The Treasury Department's Office of the Inspector General has opened an investigation into Mnuchin's travel habits.

  • Health and Human Services Secretary Tom Price

Secretary Price became the poster child of the air travel controversy with Politico's series of reports that have found he took at least 24 private charter jet flights costing the federal government about $400,000 this year. His use of private jets can be traced back to May when he missed a meeting due to a commercial flight on a day when wide-spread stormy weather caused many flights to be delayed or cancelled.

On Wednesday, President Trump said that he was "not happy" about Price's use of charter jets and indicated that he "maybe" would consider firing Price. In a statement released a day later, Price expressed regret for the flights.

"All of my political career I've fought for the taxpayers. It is clear to me that in this case, I was not sensitive enough to my concern for the taxpayer. I know as well as anyone that the American people want to know that their hard-earned dollars are being spent wisely by government officials," he said Thursday, adding that he would write the U.S. Treasury a $51,887.31 check for the cost of his seats on these flights but not for the remainder of the tab. He also swore off taking any more private charter jet flights as HHS secretary.

Hours later Politico published yet another report on Price, revealing that he used military aircraft during trips to Africa, Europe, and Asia this year, costing taxpayers an additional $500,000-plus, putting his total tab above $1 million since May. Travel on military aircraft is considered on a case-by-case basis, but typically is granted only to the president, vice president, and Cabinet members who oversee national security issues.

On Friday, Trump said that while he viewed Price as a "good man" he was concerned about the "optics" of the controversy and said a decision would be made later in the day. Not long after, Price submitted his letter of resignation.

Price's frequent use of private aircraft had been compared to how infrequently his predecessors under former President Barack Obama, Kathleen Sebelius, and Sylvia Burwell, did the same. Sebelius recently called Price's private plane travel "stunning" and admitted to using a chartered plane once. HHS officials told Politico that Burwell used a military jet to get to Havana, Cuba.

Both the HHS inspector general and the House Oversight Committee announced that they were looking into Price's travel habits and certain Democratic and Republican lawmakers have also requested more information on the matter.

  • EPA Administrator Scott Pruitt

Pruitt's trips back to his home state of Oklahoma, where he previously served as attorney general were put under a microscope after the nonprofit Environment Integrity Project released a report in July that documented Pruitt's travel using records acquired through FOIA requests.

The group said that the cost of Pruitt's trips totaled $12,000 and that he spent 43 out of 92 days from March through May in Oklahoma, or traveling to or from the state.

A probe into those trips by the EPA's Office of Inspector General was announced in August to determine the cost of Pruitt's flights to Oklahoma and elsewhere, and whether proper procedures followed were sufficient to prevent "fraud, waste and abuse with the administrator's travel that included trips to Oklahoma."

Separately, reports came out this week that Pruitt opted to use a private plane and military jet for trips over the summer. In defending each of the instances, the EPA cited logistical concerns.

  • Interior Secretary Ryan Zinke

Secretary Zinke became the latest Cabinet-level official to become involved in the air travel controversy when Politico reported Thursday evening that he traveled via private jet or military aircraft -- including a $12,000 charter plane flight -- to an event in his home state of Montana and between two islands in the Caribbean.

Interior Department spokeswoman Heather Swift said private or military aircraft were used in lieu of commercial flights because of scheduling concerns. The flights were "pre-cleared by career officials in the ethics office," she added.

Further adding fuel to the fire, a follow-up report from the Washington Post said documents showed Zinke took a flight over the summer from Las Vegas to Montana, his home state, on a plane owned by oil-and-gas executives.

Zinke told an audience at a Heritage Foundation event on Friday that the hoopla over his private jet use was "a little B.S."

Later in the day, a watchdog group asked the Interior Department's inspector general to investigate whether Zinke violated the law by chartering private planes for purposes that may represent conflicts of interest.

Beyond these four, only one other Trump Cabinet official has been scrutinized for her use of a private plane: Education Secretary Betsy DeVos. However, DeVos, who is worth $1 billion according to Forbes, reportedly covers the cost herself when she makes trips around the country on her personally owned private plane.

Also, Veterans Affairs Secretary David Shulkin and his wife, accompanied by other VA officials, attended a tennis match at Wimbledon, toured Westminster Abbey, and cruised the Thames River as part of a taxpayer-funded, 10-day work trip to Europe this summer. However, when it comes to flights, the Washington Post report on his itinerary said that the VA secretary traveled in coach on a commercial flight during at least one part of the trip.

CREW has issued FOIA requests not only for travel-related documents for Mnuchin, Price, Pruitt, and Zinke, but also at least two other top Trump officials: Agriculture Secretary Sonny Perdue and Housing and Urban Development Secretary Ben Carson.

When Price resigned Friday, CREW spent little time celebrating and saying that there is more work to be done.

"Secretary Price inappropriately spent hundreds of thousands of dollars of taxpayer money on unnecessary private jet travel. Were he not caught, he would probably still be doing so today. After being caught, he offered to pay back only a small fraction of what he owed. While his resignation ends his time in the government, it does not end the private jet scandal that others in the Trump administration, including Mnuchin, Pruitt and Zinke, find themselves in," CREW said. "Nor does it address the fact that this is a result of the tone set by the President when he decided to maintain his massive global business despite the significant conflicts of interest it causes every day and to spend many days at his own properties at great cost to taxpayers. This administration seems to believe that the government and the taxpayers serve them rather than the other way around."

Furthermore, CREW wants to know whether Carson used federal funds to travel to Trump's campaign-style rally in Phoenix last month, where he delivered introductory remarks. Members of the executive branch, except the president and the vice president, using taxpayer money for political purposes is prohibited by the Hatch Act.

What is the Trump administration doing to address the controversy?

The Trump administration appears to be making adjustments to avoid further air travel controversies. In a letter sent out to heads of federal agencies by Mick Mulvaney, director of the Office of Management and Budget, he said some top administration officials will now require prior approval from White House chief of staff John Kelly to travel on government-owned, rented, leased, or chartered aircraft.