Treasury Secretary Steve Mnuchin lied about the GOP tax plan, and he lied about launching a study of it. Now he is under investigation by his own department.

The Senate’s scam to raise taxes on almost 90 million middle-class families to pay for massive breaks and loopholes for billionaires and corporations has hit unexpected stumbling blocks, with Senate Majority Leader Mitch McConnell frantically rewriting the bill hours before a vote.

But Senate Republicans are not the only ones struggling to make their plan work.

Donald Trump’s top official in charge of the effort, Treasury Secretary Steve Mnuchin, is now facing an investigation from his own department following claims he lied to the public, and to lawmakers, about an analysis of the tax scheme.

Mnuchin has repeatedly claimed that analysis by the Treasury Department shows the proposed Republican tax breaks for the wealthy would pay for themselves, by inducing companies to invest in jobs and capital that will grow the overall tax base.

But whistleblowers in the Treasury’s Office of Tax Analysis told The New York Times on Thursday that the “analysis” Mnuchin has touted to support the GOP proposal does not exist. He has not directed his department to even do the work that he claimed they were doing.

Following the Times report, Massachusetts Sen. Elizabeth Warren sent a furious letter to the Treasury’s Office of the Inspector General, demanding answers.

“Either the Treasury Department has used extensive taxpayer funds to conduct economic analyses that it refuses to release because those analyses would contradict the Treasury Secretary’s claims, or Secretary Mnuchin has grossly misled the public about the extent of the Treasury Department’s analysis,” she wrote. “I am deeply concerned about either possibility.”

Treasury Inspector General Eric Thorson agreed. On Warren’s request, he has opened a probe into whether Mnuchin’s study exists.

Incidentally, Mnuchin’s claims that the tax plan will spur investment are at odds with numerous CEOs who willingly admit that, if the tax bill becomes law, they will pass the untaxed profits directly to shareholders — not to workers, as Trump and his fellow Republicans have insisted.

Moreover, an analysis by the Joint Committee on Taxation finds that even if the plan does grow the economy, it will still add $1 trillion to the deficit.

It is one thing for Republicans to be irrationally optimistic about trickle-down economics, which has defined their policy for decades, despite its obvious failure. But it is quite another for Republicans to falsify or cover up information to defend their plan. This takes the tax scheme from the realm of bad policy to the realm of public scandal, and the American people deserve immediate answers.