The Zurich-based bank, set up as a silk trader in 1750, has come under investigation despite a recent Swiss-U.S. deal to allow some Swiss banks to pay fines to avoid or defer prosecution over tax evasion by their U.S. customers.

That deal was a step forward in a long-running U.S. drive to pierce the shroud of Swiss bank secrecy and applies to about 100 second-tier Swiss banks, which could have to disclose information and face penalties of up to 50 percent of assets they managed on behalf of wealthy Americans.

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The agreement did not, however, cover a first group of banks already under U.S. criminal investigation including Credit Suisse, Julius Baer and state-backed regional bank Zuercher Kantonalbank.

"We knew we would have to go to U.S. authorities, so we began to prepare the documentation concerning our U.S. business some months ago, believing we would be in the second group," said Rahn & Bodmer partner Christian Rahn by telephone.

"It is difficult to evaluate whether being in the first group of banks is better for us or not."

He said the bank was informed last week that it was under investigation.

The bank will not need to set aside capital to meet regulatory requirements if it has to pay a fine, as it has provisions which will cover a potential payment to the United States, Rahn said.

Credit Suisse and Julius Baer said in July they were preparing information on client withdrawals demanded by U.S. investigators to help pinpoint tax evasion, which may have fingered other banks.

However, Rahn said he did not believe his bank was being investigated as a result of data in the so-called leavers lists, as it had stopped accepting undeclared U.S. assets in 2008 and advised clients with such assets to make voluntary disclosures to the U.S. authorities.

According to its website, the bank was established in 1750, and at the end of 2012 managed client assets of 12.5 billion Swiss francs ($13.4 billion). It employs around 200 people.

It is the oldest bank remaining in the German-speaking part of Switzerland after St. Gallen based Wegelin shut its doors earlier this year following an indictment and fine by U.S. authorities for conspiring to help U.S. clients evade taxes.

UBS paid a fine of $780 million in 2009 and delivered the names of more than 4,000 clients to avoid indictment, giving the U.S. authorities the information that has enabled them to pursue other Swiss banks.

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