Apple CEO Tim Cook can expect a tough grilling about his company’s tax practices tomorrow when he appears before the U.S. Senate Permanent Subcommittee on Investigations hearing on “Offshore Profit Shifting and the U.S. Tax Code.” An 18-month bipartisan investigation — whose findings were summarized in a memo released today — charges that Apple’s offshore subsidiary, Apple Operations International, reported net income of $30B from 2009 to 2012 but “declined to declare any tax residence, filed no corporate income tax return, and paid no corporate income taxes to any national government for five years.” In addition, Ireland-based Apple Sales International generated $74B in sales income over four years but allegedly “paid taxes on only a tiny fraction of that income” after the company negotiated a deal with the government that enabled Apple to pay a tax rate of less than 2% vs the statutory rate of 12%.All told, the investigation found that Apple avoided paying about $10B in U.S. taxes each year for the last four years. While these and other actions are legal, they constitute “the Holy Grail of tax avoidance,” subcommittee Chairman Carl Levin said today. “We intend to highlight that gimmick and other Apple offshore tax avoidance tactics so that American working families who pay their share of taxes understand how offshore tax loopholes raise their tax burden, add to the federal deficit and ought to be closed.” The subcommittee’s ranking Republican, Sen. John McCain, joined in calling Apple “among America’s largest tax avoiders.”

Apple rejects the charges, and will tell the subcommittee that it is “an American success story.” The prepared testimony adds that it paid nearly $6B in taxes here last year making it “likely the largest corporate income tax payer in the U.S.” The company will also note that its overseas operations accounted for 61% of its revenues last year. “Apple does not move its intellectual property into offshore tax havens and use it to sell products back into the U.S. to avoid U.S. tax, nor does it use revolving loans from [Controlled Foreign Corporations] to fund its domestic operations.”