The inspector general for the Interior Department has opened an investigation into Secretary Ryan Zinke’s travel during seven months in office, from his use of taxpayer-funded charter and military planes to his mixing of official trips with political appearances.

Nancy K. DiPaolo, a spokesperson for Deputy Inspector General Mary Kendall, described a broad investigation into Zinke’s “travel in general,” including “modes of travel, costs and schedules.”

"It's not just one trip," she said. "It's seven months of travel." She said the probe was prompted by numerous complaints from the public and in recent newspaper articles. Reps. Raúl M. Grijalva (Ariz.) and A. Donald McEachin (Va.), the top Democrats, respectively, on the House Committee on Natural Resources and the panel's subcommittee on oversight and investigations, also requested an investigation.

Zinke, a former Navy SEAL commander and former congressman from Montana, is one of several Trump administration Cabinet secretaries whose travel is under scrutiny. Tom Price, another former member of Congress who was Secretary of Health and Human Services, resigned Friday after taking at least $400,000 in chartered flights at taxpayer expense. The watchdog for the Environmental Protection Agency also is investigating agency head Scott Pruitt's frequent travels to his home state of Oklahoma.

In June, Zinke and his staff chartered a four-hour private flight from Las Vegas to near his home in Montana aboard a plane owned by executives of a Wyoming oil-and-gas firm, aviation and business records show.

Interior Secretary Ryan Zinke speaks during a White House briefing in April. (Jabin Botsford/The Washington Post)

The flight cost taxpayers $12,375, according to an Interior Department spokeswoman. Commercial airlines run daily flights between the two airports and charge as little as $300. Zinke’s spokeswoman, Heather Swift, said no commercial flight was available that would have worked with his schedule that night.

Zinke appeared at an event held by a major campaign donor, giving a motivational speech at a dinner for the Las Vegas Golden Knights, a hockey team owned by Bill Foley, the chairman of Fidelity National Financial. Employees and PACs associated with the financial services company have given close to $200,000 to Zinke’s past congressional campaigns.

“Claims that the Secretary’s full schedule required the use of chartered aircraft deserve scrutiny,” the lawmakers wrote in a letter to Kendall last week. “It appears as though Secretary Zinke and his staff could have taken a commercial flight from Las Vegas to Montana if he did not attend the motivational speech to the hockey team owned by his friend and campaign contributor.”

Zinke was in the Las Vegas area that day after flying on a commercial Southwest Airlines jet from Reno, Nev., where he spoke the night before at a nearby dinner in Lake Tahoe held by the Rule of Law Defense Fund, a conservative group of attorneys general backed by the Koch brothers.

Just before the Golden Knights dinner, Zinke had appeared in the tiny rural Nevada town of Pahrump to announce a routine local funding grant from Congress to rural communities. It was one of several official trips that coincided with weekends Zinke spent at his homes in Santa Barbara, Calif., and Montana.

The secretary and his official entourage also boarded private flights between the Caribbean islands of St. Thomas and St. Croix during a three-day trip to the Virgin Islands in March, his first month on the job.

Swift said Zinke’s charter flights were authorized by ethics officials and booked only when feasible commercial flights were unavailable. She said previous interior secretaries flew charter flights when needed.

The watchdog group Campaign for Accountability also requested an investigation into whether Zinke violated the federal Hatch Act by speaking to the hockey team as part of his official duties. The act prevents executive branch employees from engaging in partisan political activity.

It is unclear whether taxpayers paid the costs of Zinke’s political appearances.

In remarks on American energy independence at the Heritage Foundation last Friday, Zinke called the media scrutiny “a little B.S.” and defended flights he has taken on military and charter aircraft since he took office.

“I believe taxpayers absolutely have a right to know about official travel costs,” he said, and listed three occasions when he used charter planes to fly to Alaska for a bipartisan trip arranged by Senate Energy and Natural Resources Committee Chair Lisa Murkowski (R-Alaska), the flight to Montana and trips between the Virgin Islands.

Zinke did not mention the costs of those flights or whether staffers traveled with him. Zinke also said that he has flown U.S. military jets at least twice, once at the invitation of Agriculture Department Secretary Sonny Perdue and again at the invitation of President Trump.

Using taxpayer funds wisely is good government, Zinke said, but “there are times when we have to use [charter flights] as an option.”