(Bloomberg) -- Swiss bank Mirabaud & Cie. is being asked by a powerful U.S. lawmaker whether it properly told the Internal Revenue Service about a businessman’s accounts that are now part of America’s biggest individual tax-evasion case.

Senator Ron Wyden, chairman of the Senate Finance Committee, asked in a letter whether Mirabaud told the IRS about the existence of accounts that benefited Robert Brockman, a Houston software tycoon indicted on charges of evading taxes on $2 billion in income over two decades. Swiss prosecutors froze about $950 million in Brockman accounts at Mirabaud after his indictment last October.

“I am deeply concerned by the duration and scale of this alleged tax evasion scheme, particularly the ability to hide billions of dollars offshore from the IRS for such an extended period of time,” including in accounts at the bank, Wyden, an Oregon Democrat, wrote on Wednesday.

The bank said in an emailed statement it has “cooperated fully with all requests for information made by the U.S. authorities through established bilateral legal assistance and exchange of information channels” and it has “fully implemented procedures” to comply with requirements on reporting about U.S. account holders.  

The indictment against the billionaire “makes clear that Mr Brockman went to great lengths to deceive Mirabaud about his real involvement in the accounts held with the bank, none of them were in his name,” the bank said. 

Brockman, 80, is accused of using a web of Caribbean entities, as well as Bermudian and Swiss bank accounts, to hide money from the IRS. After he pleaded not guilty, his lawyers said he has dementia and can’t aid in his defense. A federal judge in Houston will decide whether Brockman is competent to stand trial. Mirabaud is not charged with wrongdoing in the indictment, which is the largest U.S. tax evasion case ever against an individual.

Read More: Houston Tech Mogul Indicted for ‘Largest-Ever Tax Charge’ (3)

Wyden asked if Mirabaud verified Brockman’s status as a U.S. taxpayer, and if it queried whether he filed reports of foreign bank and financial accounts, required by U.S. law. The letter also poses several questions about Mirabaud’s compliance with the Foreign Account Tax Compliance Act, which requires overseas financial institutions to report assets held by U.S. account holders.

Brockman, the former chief executive officer of Reynolds & Reynolds, is accused of several large transfers of untaxed proceeds from investments to Mirabaud, including $799 million in 2010.

In recent months, Wyden has asked other companies about their role in tax investigations, including Credit Suisse Group AG, Caterpillar Inc. and AbbVie Inc. Wyden is seeking answers from Mirabaud by Oct. 1.


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