The agreement could be a big step toward resolving a long-running dispute between the two countries. Swiss banks would be able to turn over customer data without violating that country's bank secrecy laws. At the same time, those banks could pursue legal settlements with the U.S. Justice Department and avoid criminal prosecution.
But to do so, the banks would have to disclose reams of information about U.S. clients, including those who may have moved money out of the banks after the U.S. began its crackdown in 2009.
The agreement "enhances the department's ability to follow the money, both within Switzerland and globally," the official said.
It may also encourage more Americans to come forward and admit past violations, the official added. Thousands have already done under a voluntary program to avoid criminal charges for tax evasion.
"They are close to out of time and need to do so now," the official added.
The crackdown began in 2009 after a whistleblower notified U.S. authorities that UBS, the largest Swiss bank, was enabling Americans to evade taxes.
UBS settled the case later that year. It turned over account records on 4,500 U.S. customers and paid a $780 million fine.