ProPublica said that it doesn't know the source of its secret tax information on the super-rich, and admitted that a hostile state could have sent it

• ProPublica said it does not know the identity of the source who passed them a trove of IRS files.
• The outlet published a bombshell investigation into the tax practices of the mega-rich.
• ProPublica said it is confident of the material even if the source had hostile motives.

ProPublica says it does not know the identity - or motivations - of the source who passed it private tax documents for its recent bombshell investigation of the ultra-rich.

On Tuesday, the left-leaning outlet published its a flagship investigation based on the private IRS documents of billionaires such as Jeff Bezos, Elon Musk, Michael Bloomberg and Warren Buffett.

The federal government is now investigating the origins of the leak, which it says was illegal. In an accompanying article, ProPublica's CEOs Stephen Engelberg and Richard Tofel revealed that they don't know who sent the documents.

"We do not know the identity of our source," they wrote. "We have considered the possibility that information we have received could have come from a state actor hostile to American interests."

Engelberg and Tofel said the source told them they were motivated by ProPublica's previous reporting on taxation, but, the CEOs said, "we do not know for certain that is true."

Noting recent hacks on US servers from hostile states, they added: "We have long held that those motives are irrelevant if the information is reliable."

They said that they had vetted the information extensively by comparing the documents to those they already had access to.

"In every instance we were able to check - involving tax filings by more than 50 separate people - the details provided to ProPublica matched the information from other sources," they wrote.

The ProPublica story revealed the actual rates of tax the ultra-rich pay, as well as a wide array of details of their tax practices.

Explaining why they published the private information, Engelberg and Tofel argued that it is in the public interest to reveal the tax avoidance practices of the billionaires, who were often found to have paid proportionately much less than regular Americans.

Tactics highlighted by ProPublica include low-interest borrowing to keep tax bills down, and an allegation that Bezos claimed a tax credit designed for much lower-income households than his.