TRUMP: It's 'fake news' that a reporter just opened his mailbox and found my tax returns

President Donald Trump attacked an MSNBC report on his tax returns Wednesday morning, calling it "FAKE NEWS!" that a reporter found the president's tax returns in his mailbox.

"Does anybody really believe that a reporter, who nobody ever heard of, 'went to his mailbox' and found my tax returns? @NBCNews FAKE NEWS!," Trump tweeted.

Notably, Trump does not seem to question the story itself, as the White House confirmed part of the president's 2005 tax returns ahead of a Tuesday night's report from MSNBC host Rachel Maddow.

The reporter, David Cay Johnston, - who the president says "nobody ever heard of" - won a Pulitzer Prize in 2001, and wrote a biography of Trump, "The Making of Donald Trump," in 2016.

Johnson's publisher, Melville House, notes on the website for his Trump biography that the book is "the culmination of nearly 30 years of reporting on Donald Trump" and the reporter "has covered Trump more closely than any other journalist working today."

Johnston suggested on Maddow's show that Trump himself could have leaked the documents to him.

Images of the 1040 forms were marked with a "Client Copy" stamp that suggested the documents may have been leaked by someone close to Trump, if not Trump himself.

Maddow herself acknowledged on her program that the tax returns alone weren't an important story, but that the fact that it was leaked to a reporter, was.

David Choi contributed reporting to this story.