Details claiming to reveal how little income tax US billionaires pay have been leaked to an news website.
ProPublica says it has seen the tax returns of some of the world's richest people, including Jeff Bezos, Elon Musk and Warren Buffett.
The website alleges Amazon's Mr Bezos paid no tax in 2007 and 2011, while Tesla's Mr Musk paid nothing in 2018.
A White House spokeswoman called the leak "illegal", and the FBI and tax authorities are investigating.
ProPublica said it was analysing what it called a "vast trove of Internal Revenue Service data" on the taxes of the billionaires, and would release further details over coming weeks.
While the BBC has not been able to confirm the claims, the alleged leak comes at a time of growing debate about the amount of tax paid by the wealthy and widening inequality.
ProPublica said the richest 25 Americans pay less in tax - an average of 15.8% of adjusted gross income - than most mainstream US workers.
Jesse Eisinger, senior reporter and editor at ProPublica, told the Today Programme: "We were pretty astonished that you could get [tax] down to zero if you were a multi-billionaire. Actually paying zero in tax really floored us. Ultra-wealthy people can sidestep the system in an entirely legal way."
"They have enormous ability to find deductions, find credits and exploit loopholes in the system," he said.
So while the value of their wealth grows enormously through their ownership of shares in their company, that's not recorded as income.
But there's more than that, he said: "They also take aggressive tax deductions, often because they have borrowed to fund their lifestyle."
He said US billionaires buy an asset, build one or inherit a fortune, and then borrow against their wealth.
Because they don't realise any gains or sell any stock, they're not taking any income, which could be taxed.
"They then borrow from a bank at a relatively low interest rate, live off that and can use the interest expenses as deductions on their income," he said.
