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

During an appearance on Ms. Maddow’s show Tuesday night, Mr. Johnston speculated that Mr. Trump might have leaked the pages of his own tax returns even though he was the first modern presidential candidate to refuse to release them.

“It’s entirely possible that Donald sent this to me,” Mr. Johnston said, though he added that he did not know whether that was the case. “It’s a possibility, and it could have been leaked by someone at his direction.”

Ms. Maddow trumpeted the returns as “breaking news” on Twitter an hour before her 9 p.m. show, raising expectations that her report would reveal striking financial news about the president.

Instead, the two pages largely confirmed that Mr. Trump made a significant amount of money in 2005 and paid millions in taxes after writing off about $100 million in business losses that reduced the overall tax payment.