The disclosure of tax records on rich Americans is being investigated by US authorities.
The files, which showed that some of the wealthiest people paid little or no income tax, were leaked to media outlet ProPublica.
The US Treasury Department has asked law enforcement agencies to probe the case.
New York-based journalism nonprofit, ProPublica, said that it obtained "a vast trove of Internal Revenue Service data on the tax returns of thousands of the nation's wealthiest people, covering more than 15 years".
The bombshell report showed that Amazon founder Jeff Bezos paid no income tax in 2007 and 2011, while Tesla chief Elon Musk avoided all payments in 2018.
ProPublica said the billionaires did nothing illegal in their tax declarations, but employed tax-avoidance strategies "beyond the reach of ordinary people."
They also benefited from the way taxable income is defined in the US tax code.
The current system does not take into account the growing value of assets such as stock and property. It only taxes proceeds from the sale of those items.
The report comes days after G7 finance ministers endorsed a global minimum corporate tax rate of at least 15%.
This is one of several tax proposals aimed at ensuring profitable multinationals pay their fair share.
Democratic senator Elizabeth Warren is proposing an initiative to tax the super-wealthy that would include the value of their stock holdings and homes, rather than focusing only on their income.
Ms Warren said on Twitter: "Our tax system is rigged for billionaires who don't make their fortunes through income, like working families do."
