ProPublica does not know whom to thank for providing the raw material for what organization president Richard Tofel called “the most important story we have ever published.”

On Tuesday morning, the nonprofit investigative news operation published “The Secret IRS Files,” the first in a series of investigative stories based on federal tax documents from thousands of wealthy individuals covering a period of more than 15 years. It lays out how wealthy Americans “exploit the structure of our tax code to avoid the tax burdens borne by ordinary citizens,” with many not paying taxes at all for certain years.

In a companion piece, Tofel and top editor Stephen Engelberg made a stunning revelation: ProPublica does not know who sent the documents or why they were sent.

“The source says they were motivated by our previous coverage of issues surrounding the IRS and tax enforcement,” they wrote, “but we do not know for certain that is true.”

They acknowledged the possibility that the documents could have come from “a state actor hostile to American interests.” But “provenance is not essential; accuracy is.”

In an interview, Engelberg said ProPublica was further convinced of the veracity of the documents after cross-checking the data with some of the individuals to whom the tax returns belonged. “It was certainly relevant that in each case that we were contacting people, we were getting the same numbers they had, so it became pretty clear that we had an accurate set of things,” he said. Amazon founder and chief executive Jeff Bezos, who also owns The Washington Post, was among the wealthy Americans covered in the story.

Tofel and Engelberg said they wrestled with the ethics of publishing private tax documents but decided that it was in the public interest to do so.

Some skeptics said the report was noteworthy but did not fundamentally change the perception of American billionaires as savvy employers of tax code workarounds and enterprising accountants.

The bigger question, at least among journalists, seemed to focus on where the documents came from and who would have access to them. The Internal Revenue Service is also interested in how the documents were acquired by the source, announcing an investigation on Tuesday.

The Treasury Department asked law enforcement authorities to investigate the disclosure of tax records cited in a media report on June 8. (Reuters)

Engelberg said ProPublica was approached directly by the source, though he said the organization is “trying to be a little bit vague about that just so that we don’t give any hints.” He acknowledged that the publication has had “sufficient conversation” with the source.

Traditionally, media organizations prefer to know who is sending source material to have a better grasp of the authenticity of the material, the way in which it was obtained and the motivations of the sender. But, in this case, some journalists praised ProPublica for focusing on the veracity of the documents as the primary factor in deciding to publish, not the identity of the source.

The reporting project, he said, has been in the works for months and included a “significant investment” in the effort. Even internally at ProPublica, he said the group that began working on the material was small, though the circle widened over time.

“We certainly handled this very carefully internally. We worked pretty hard to try to make sure this didn’t become a matter of public conversation before it published,” he said. “By the end of it, the number of people involved in the room — it was a big Zoom.”

The only recent precedent is a series of New York Times stories that were based on tax documents involving President Donald Trump. Metro reporter Susanne Craig recalled in a first-person article that she received a copy of Trump’s 1995 tax records in her mailbox at the Times building in New York City. The Times was able to confirm the authenticity of the documents through the accountant who had prepared them. “Nowadays, when people are worried that anything sent by email will leave forensic fingerprints, ‘snail mail’ is a great way to communicate with us anonymously,” Craig wrote. Later, the Times obtained a much larger trove of Trump’s tax information.

At a time when multiple individuals have been imprisoned recently for leaking government documents to reporters — including a former Treasury Department official who was sentenced last week to six months in prison — Engelberg said the publication “wants to be very protective of our source” and “as meticulous and careful as we possibly could be.”

The “Secret IRS Files” is likely to mark a new chapter for ProPublica, which has received six Pulitzer Prize awards since it began publishing in 2008. The organization employs 168 people, including about 140 in the newsroom.

“It’s one of the biggest things that we have ever done, if not the biggest,” Engelberg said. “We anticipate covering this for many, many months, if not years. We have really sort of scratched the surface of what needs to be said and drawn from this material.”

The publication has also solicited reader assistance, specifically asking for those “knowledgeable about tax law, accounting, the IRS, wealth management or tax policy of any kind” to help with reporting on future stories.

Engelberg, who spent 18 years with the Times, said the document leak was unlike anything he’s experienced before, including his eight-year tenure as ProPublica’s editor in chief. “Very few times do people send you things,” he said, “but they tell you things all the time.”