"Using this information to identify U.S. clients would appear to be more productive than simply pursuing agreements and treaties with the Swiss," Grassley added.
The United States first issued a summons for approximately 52,000 UBS client names in July of 2008, which the Swiss blocked.
A settlement was then reached in August of 2009, when the United States agreed to give up its right to all 52,000 names in exchange for getting prompt access to key account holders, which totaled 4,450. On Tuesday, Swiss lawmakers voted to block that treaty. However, that decision could be overturned in the future.
To get a composite of UBS clients living in the U.S., Grassley has requested that Treasury officials compile a detailed list of all the steps the IRS has taken with the information provided by Birkenfeld and if the statute of limitations would preclude the agency from using the data.
"It has been over three years since Mr. Birkenfeld approached the Department of Justice, IRS and the Securities and Exchange Commission about potential tax evasion facilitated by UBS AG on behalf of U.S. clients," Grassley wrote. "I am worried that the Internal Revenue Service is doing next to nothing to identify tax evasion by U.S. taxpayers utilizing these accounts."
Grassley seeks a response from the Treasury to his requests by June 18.