Interior Secretary Ryan Zinke was found to have used taxpayer funds for expensive, private charter flights even when commercial options were available – the fourth member of the Trump administration identified as having done so.

Zinke and members of his staff flew on two chartered planes round trip from St. Croix and St. Thomas in the Caribbean on March 31, and he flew with several of his flew from Las Vegas to his home state of Montana on June 26, Politico reported Thursday.

"All of this travel was done only after it was determined … that no commercial options existed during the promulgated schedule," Zinke said Friday ahead of an address on energy at the Heritage Foundation. The flights, he said, were "only booked after extensive due diligence by the department's" ethics and legal teams.

Two private jets were charted by the Interior Department to fly Zinke and his staff from St. Croix to St. Thomas to celebrate the 100th anniversary of the sale and turnover of the U.S. Virgin Islands from Denmark, before returning to St. Croix the same night.

Swift told Politico other arrangements were not available, but travel websites show frequent daily flights between the two islands for around $200 for the flight each way, which takes about a half hour. The cost of the charters has not been made public.

The flight to Montana cost of $12,375, Interior Department spokeswoman Heather Swift told Politico, and was booked with pre-approval from the department's ethics office after officials could not find flights that accommodated Zinke's official schedule. The plane, co-owned by oil and gas exploration firm Nielson & Associates executive vice president Jay Nielson, was charted through a private firm called Choice Aviation, according to The Washington Post.

Zinke's travel to Nevada included a public lands announcement, followed by a speaking engagement with the Vegas Golden Knights, the city's new NHL hockey team. Zinke, a former Republican congressman from Montana, then flew to Glacier Park International Airport in Kalispell, Montana. He stayed overnight at his home in Whitefish, about 20 minutes from Kalispell, before attending the Western Governor's Association annual meeting. He participated in a GQ photoshoot and gave an interview to Outside Magazine while before flying commercial air back to Washington, D.C., with his staff, according to Politico's report.

While only a few direct flights take place each week between Las Vegas and Kalispell, travel planning websites list multiple daily flights from Las Vegas to Kalispell with stops in Salt Lake City or Seattle that cost only a few hundred dollars.

Zinke also used military aircraft for travel on several occasions, including a trip to Norway in May with his wife, Lolita, before flying to Alaska. While in Alaska, Zinke participated in Senate Energy and Natural Resources Committee events, for which chartered planes were used to travel within the state. The department said the Zinkes have reimbursed the government for the costs of Lolita Zinke's travel.

Zinke is the fourth senior member of Trump's administration – along with Secretary of Health and Human Services Tom Price, Treasury Secretary Steve Mnuchin and Environmental Protection Agency Administrator Scott Pruitt – who are under scrutiny for their use of taxpayer funds for private travel.

The cost of known private travel arrangements for Health and Human Services Secretary Tom Price exceeds $1 million, according to Politico's investigation.

In addition to around $400,000 worth of domestic travel, Price racked up travel expenses exceeding $500,000 on trips overseas to destinations in Europe and Asia.

Price traveled aboard a Department of Defense-owned Gulfstream C-37B in May on a weeklong trip through Liberia, Germany and Switzerland, where he attended the World Health Assembly in Geneva, at a total cost of $311,418.25, according to an invoice reviewed by Politico. Flying military aircraft requires White House approval.

Price's wife, Betty, and eight HHS staff joined him aboard the private jet on the leg between Berlin and Geneva, which cost $16,000 in taxpayer funds. Commercial flights along that route typically cost around $250.

He has also visited Beijing, Ho Chi Minh City and Tokyo on military aircraft.

After Trump on Wednesday said he was "not happy" with the cost of Price's travel, the secretary, a former Republican congressman from Georgia who had previously criticized government use of private jets, apologized.