A report in the New York Times says a $916 million loss in the '90s might have allowed Donald Trump to legally avoid paying any income taxes for almost two decades. (Sarah Parnass/The Washington Post)

When New York Times reporter Susanne Craig checked her office mailbox a few days ago, a thin Manila envelope immediately caught her eye. She almost gasped when she opened it.

“I thought it was a hoax,” she said Sunday. “My reaction was, ‘No way this is real.’ ”

The typed return address read “The Trump Organization.” Inside were three photocopied pages Craig realized could be dynamite: They appeared to be from Donald Trump’s 1995 tax returns.

Those were the decidedly low-tech beginnings of what may turn out to be one of the most consequential stories of the 2016 presidential campaign. Late Saturday, the Times revealed that Trump had declared a $916 million loss in 1995, wiping out any federal taxes that year and setting himself up to avoid 18 years of similar obligations.

The story, which Trump’s campaign did not contest or confirm, filled in one bit of the mystery surrounding the real estate mogul’s taxes. Trump has repeatedly declined to release his most recent returns, prompting his rival for the presidency, Democrat Hillary Clinton, to suggest he was hiding information that could hurt his candidacy.

The New York Times published portions of Donald Trump's tax return from 1995 suggesting that he lost $916 million. The Post's Margaret Sullivan and Callum Borchers react to the validity of the Times' source and the risks the paper took in Trump's returns. (Erin Patrick O'Connor/The Washington Post)

The Times’ story was a rare animal, a bombshell based on a source whose identity is unknown even to people at the news organization that broke the story. Although anonymous sources are commonly used by journalists to elicit sensitive information, reporters almost always know their identity, even if they don’t disclose their names to readers or viewers.

That doesn’t appear to be the case in the Times’ story, which carried the bylines of four reporters, including Craig and David Barstow, an investigative reporter who has won three Pulitzer Prizes.

While Craig declined to discuss her understanding of who sent the Trump documents, Times deputy executive editor Matt Purdy was definitive: “We do not know the identity of the source.”

In hindsight, however, that may have been among the least problematic elements behind the documents Craig received that Friday, Sept. 23. The major challenge was authenticating the three pages and placing them in the proper context to understand Trump’s tax strategy at the time, said Dean Baquet, the newspaper’s executive editor.

The documents “looked real,” he said. “But who knew?”

The Times described the documents as the first pages of three filings: a New York state resident income tax return, a New Jersey nonresident tax return and a Connecticut nonresident tax return.

Among the troubling aspects was a line on one of the forms bearing the nine-figure sum Trump claimed as his personal loss. The figure’s first two digits — 9 and 1 — were typed onto the form in a different font than the digits making up the rest of the number, noted reporter Megan Twohey.

This raised the possibility that the documents could be fakes, just as the unusual typescript in documents purported to be part of President George W. Bush’s military records was called into question after CBS News used them in a “60 Minutes II” story in 2004. (Those documents were never definitively shown to be bogus, but the suspicions they raised eventually led to the firings, resignations or early retirement of people involved in the CBS story, including anchor Dan Rather.)

In addition to corroborating publicly available information contained on the forms, such as Trump’s Social Security number, the Times hired several tax experts to review the documents. They suggested that the documents were in line with accounting permissible under the federal tax code in 1995.

The key to authentication was a semi-retired accountant named Jack Mitnick, who had prepared and signed Trump’s 1995 return. Barstow tracked down Mitnick in South Florida and “over coffee and bagels,” as Craig put it, confirmed that Mitnick had prepared them.

Mitnick also explained the mysterious 9 and 1, telling Barstow the two digits had to be hand-typed onto the tax form because they kept being wiped off the line when transmitted from an electronic tax-preparation program.

Based on Mitnick’s comments and other background material gathered by Barstow, Craig, Twohey and reporter Russ Buettner, Baquet decided the story was ready for publication.

But just before that, according to the Times, a lawyer for Trump, Marc E. Kasowitz, emailed a letter to the paper threatening “prompt initiation of appropriate legal action” if the newspaper published the private documents.

Trump’s campaign did not dispute the documents’ authenticity or question the Times’ conclusions. It instead issued a statement that indirectly confirmed the story, reading, in part, “Mr. Trump is a highly-skilled businessman who has a fiduciary responsibility to his business, his family and his employees to pay no more tax than legally required. Mr. Trump knows the tax code far better than anyone who has ever run for president and he is the only one that knows how to fix it.”

Baquet, interviewed Sunday morning, expressed no regrets. “There’s no more public figure than the president and no more public endeavor than running for president,” he said. “Given what he has said about taxes and what he won’t show about his own, it’s important for voters to have this information.”

As for Craig, she’s still guessing why the source chose to send her the envelope.

Some of it might be her experience covering Wall Street for a decade or so for the Wall Street Journal and the Times. Part of it might be her coverage of Trump’s business career for the Times over the past nine months, including an investigation this summer of Trump’s holdings that revealed his businesses are carrying more than twice as much debt as Trump has publicly disclosed.

In any case, Craig said she’d welcome more Manila envelopes from her source.

“I sit right by the mailboxes, and I’m constantly checking mine,” she said. “You never know what’s going to be in there.”

Correction: An earlier version of this report misspelled the name of New York Times reporter Russ Buettner.