Zinke's use of Interior Department and other helicopters was revealed earlier this year when the department released his office and travel schedules in response to Freedom of Information Act requests. Documents show that helicopter trips to and from events within a few hours of his Washington office cost taxpayers more than $14,000.
first reported on the flights.
Sen. Maria Cantwell, a Democrat from Washington state, called for the investigation to include Zinke's "taxpayer-funded helicopter rides."
"If he misused public funds, he should write the Treasury a check for the full amount & apologize," she said.
The Interior Department Inspector General's Office declined to say Friday whether its investigation, which began earlier this year after Zinke's trips to the US Virgin Islands and Las Vegas combined work with political events, includes the helicopter uses.
"We are taking a comprehensive look at the secretary's travel since he took office," said Nancy DiPaolo, the inspector general's spokeswoman.
Interior spokeswoman Heather Swift said one flight was necessary for Zinke to attend "an official congressional event" -- the swearing-in of his successor, Montana Republican Greg Gianforte, to the US House -- "before going to an emergency management briefing."
Swift described the story, based on official documents, as "yellow journalism."
"It's complete garbage and yellow journalism at its worst. After a congressional hearing, the secretary attended an official congressional event with the congressman from Montana and the speaker of the House before going to an emergency management briefing. On the other occasion he did an aerial survey of a power line corridor, which was pending before the Department, while traveling back from meetings in southern Virginia in order to get to a meeting with the Vice President."
Zinke is one of several Trump administration Cabinet officials who have come under scrutiny for their travel habits since Tom Price racked up half a million dollars in flights on private jets as health and human services secretary. Price resigned over the controversy in September, though he and his aides said the trips had been approved through the usual legal and ethics offices at HHS.
Travel records show Zinke used government and state-owned helicopters on other occasions, such as when touring national monuments during his review of federally owned land.