NEW YORK (Reuters) - A Swiss asset management firm agreed to pay $5 million to settle charges it helped American clients evade U.S. taxes by opening undeclared foreign bank accounts for a decade, the U.S. Department of Justice said on Tuesday.

Prosecutors said they entered a non-prosecution agreement with Prime Partners SA rather than pursue criminal charges because of the Geneva-based firm’s “extraordinary cooperation.”

They said this included the production of roughly 175 unredacted client files, and voluntary implementation of remedial measures even before investigators began their probe of undeclared accounts, dating from 2001 to 2010.

Prime Partners oversaw $270 million of assets for U.S. taxpayer clients in 2008, and ended last year with $2.6 billion of assets under management, settlement papers show. The firm was founded in 1998.

A lawyer for Prime Partners did not immediately respond to a request for comment.

Under the settlement, Prime Partners will forfeit $4.32 million of fees and pay $680,000 representing unpaid taxes resulting from its clients’ conduct.

Prime Partners “created sham entities and even counseled their clients to use pay phones and prepaid debit cards to avoid detection of their tax fraud scheme,” Acting U.S. Attorney in Manhattan Joon Kim said in a statement.

The settlement “should serve as proof that cooperation has tangible benefits,” Kim added.

U.S. authorities and the Internal Revenue Service have pursued a crackdown on offshore tax evasion for several years.