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US media outlet ProPublica said 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".

Treasury Department spokeswoman Lily Adams said in an emailed statement the matter had been referred to the FBI, federal prosecutors and two internal Treasury Department watchdogs, "all of whom have independent authority to investigate".

The Internal Revenue Service (IRS), part of the Treasury Department, is the nation's tax-collecting agency and many tax records are considered confidential documents.