The Trump administration won't come clean about the details of taxpayer-financed private travel by Interior Secretary Ryan Zinke and his wife, Lolita Zinke. Now they're being sued for it.

The scandal involving taxpayer-financed travel on private airplanes by Cabinet members continues to grow, as the Trump administration is now being sued for covering up the details of such trips involving a Cabinet secretary and his wife.

American Oversight, a watchdog group, filed a lawsuit against the Department of the Interior for its refusal to be fully transparent about details surrounding Interior Secretary Ryan Zinke’s travels with his wife, Lolita Zinke, on board privately chartered flights.

Those trips are now under investigation by the government office of the Inspector General, the internal watchdog for waste, fraud, and abuse.

American Oversight filed a Freedom Of Information Act (FOIA) request with the department, attempting to uncover the details and extent of Lolita Zinke’s role in the travel plans, and within the government. The organization is interested in information that would explain “her participation in internal meetings, presence on official trips, and emails sent be her in relation to government business.”

But the response from the government to the FOIA was inadequate, prompting the lawsuit.

“We’re suing the Department of the Interior to uncover the truth about what role Secretary Zinke’s wife is playing in running the department, how much it’s costing taxpayers, and why she was jetting off to Europe with her husband on a military plane,” American Oversight Executive Director Austin Evers said in a statement.

According to congressional filings as reported by Forbes, (Zinke previously served in Congress representing Montana), the Zinkes are worth $1.9 million and own multiple rental properties in his home state, a $100,000 art collection, and a 1938 Cadillac. Forbes also reports that Zinke could also have access to a family trust “worth millions more.”

The so-called “Billionaire Air” scandal has cost taxpayers well over $1 million dollars, as multiple cabinet secretaries used private jets to ferry them across the nation. So far, the heads of Health and Human Services, the EPA, Interior, Energy, and Transportation have been exposed for using these government perks.

Health and Human Services Secretary Tom Price has already resigned after news of his luxury travel embarrassed the Trump administration.

Donald Trump’s senior adviser, Kellyanne Conway, also traveled with Price, and the senior Democrat on the House Oversight Committee, Maryland Rep. Elijah Cummings, has requested details of her expenses.

To date, it is unclear how many other cabinet secretaries and advisers — along with their spouses — are involved in this misappropriation of tax dollars.

And if the Trump administration continues to try to cover up the details, this lawsuit is likely only the first of many to come.