WASHINGTON (Reuters) - The Interior Department’s watchdog agency has launched an investigation into Secretary Ryan Zinke’s travels after reports emerged last week that he had used a private plane owned by an oil executive, the inspector general’s office said on Monday.

U.S. Interior Secretary Ryan Zinke rides a boat to Georges Island, while traveling for his National Monuments Review process, in Boston Harbor, Massachusetts, U.S., June 16, 2017. REUTERS/Brian Snyder

The Inspector General (IG) opened an investigation on Friday after receiving numerous complaints on Thursday about Zinke’s travels, including the use of three chartered flights, IG spokeswoman Nancy DiPaolo told Reuters.

One of those flights taken in June from Las Vegas to near Zinke’s hometown in Montana was on a plane owned by oil and gas executives, costing taxpayers over $12,000, the Washington Post reported last week.

Reports of Zinke’s use of private and military planes came amid heightened scrutiny into private plane use by Health and Human Services Secretary Tom Price, who resigned on Friday under pressure from President Donald Trump amid an uproar over his use of costly private charter planes for government business.

EPA Administrator Scott Pruitt and Treasury Secretary Steve Mnuchin have also come under scrutiny over reports of expensive private plane use. White House budget director Mick Mulvaney on Friday advised federal officials against chartering aircraft or using government planes unless necessary, reminding them that it comes out of taxpayers’ pockets.

Zinke denied any inappropriate travel in a speech at conservative think tank the Heritage Foundation on Friday and said his travels were approved by “career employees” of the Interior Department’s ethics office.

He called reports about his recent use of chartered private flights “a little B.S.”

Zinke spoke at a dinner on June 26 to the Vegas Golden Knights hockey team, a new NHL team in Las Vegas owned by Bill Foley, chairman of Fidelity National Financial Inc, a donor to Zinke’s congressional campaigns, the Washington Post reported.

Zinke flew with staff on the chartered plane to Kalispell, Montana, near his hometown of Whitefish, where he was due the next day to address the Western Governors’ Association even as commercial flights were available.

Interior confirmed that Zinke flew two other chartered flights between the U.S. Virgin Islands. The Interior Department has jurisdiction over the Virgin Islands.

In a letter requesting an IG investigation of Zinke’s travels, the top Democrats on the Natural Resources and Oversight committees questioned the purpose of the use of taxpayer flights to attend events unrelated to his work.

“We need to know how many taxpayer-funded trips Secretary Zinke and his wife Lola have taken to serve their personal or political interests rather than the public trust,” said Congressman Raul Grijalva, who penned the letter with Congressman Donald McEachin.