Rachel Maddow has broken a blockbuster story with an exclusive interview with David Cay Johnston, a reporter who received a leaked copy of President Donald Trump’s 2005 federal tax return in his mailbox.

The two-page 1040 individual tax return shows that Trump paid $38 million in taxes based on $150 million in reported income. The White House has verified the authenticity of the document.

Johnston, who won the Pulitzer Prize for his New York Times tax reporting in 2001 and 2002, is also the author of the book The Making of Donald Trump. On Maddow’s March 14 show, Johnston pointed out that the leaked document reveals just how much Trump would benefit from repealing the Alternative Minimum Tax, which he is proposing in his tax reform plan.

The AMT is designed to prevent wealthy individuals from entirely eliminating their tax liability through itemized deductions.

“If we didn’t have the AMT in 2005 — which Trump wants to get rid of — Trump would have paid an effective rate lower than most working people,” said Johnston. “Because of AMT, he had to pay an effective rate of 24 percent, still far below the 35 percent rate in 2005.”

The leak comes as public pressure continues to mount on Trump to release all of his tax returns, which every U.S. president has done since Richard Nixon. Over one million people have signed a petition and 19 states have introduced bills calling for tax disclosure as a condition for running for the President. Trump’s justification for withholding this information throughout his campaign was that he was being audited.

The public interest in his tax returns is not for petty reasons, like whether or not Trump is really worth the $10 billion he claims. The main question is whether the president is financially beholden to a foreign power, government, or corporation.

On last night’s show, Maddow asked, “What is his relationship with foreign sources of income? Is he receiving any money through his business interests that can be traced back to foreign governments?”

“There’s a reason Donald Trump is hiding something,” said Cay Johnston. “He is desperate to keep something from the public. We should be concerned about his connection to Russian oligarchs, a state-sponsored criminal network. In Trump’s case, there are too many intersecting business interests, including with Deutsche-Bank, which recently started to be investigated by the New York Attorney General (who Trump just fired).”

“We want to see the sources of income,” Johnston continued. “We want to know who the partners are. We want to know who he’s paying interest — who he owes money. He owes a lot of money to the Bank of China, which also leases sections of Trump Tower.”

Johnston will no doubt be checking his mailbox regularly from now on.

View the full Rachel Maddow segment with David Cay Johnston.