A day after announcing a quarterly loss of more than $17 billion, as well as plans to cut a wide swath through the ranks of its investment bank, it is clear that UBS, the giant Swiss bank, has another big problem on its hands.

U.S. prosecutors and regulators are investigating whether UBS advisers helped American clients evade U.S. taxes from 2000 to 2007.

The bank said today that U.S. authorities had briefly detained an employee when he entered the country, and he is still in the United States.

"Our understanding is that the respective employee, who has not been charged with any wrongdoing by the U.S. government, will remain in the United States pending discussions with the U.S. authorities regarding resolution of his status as a witness," the bank said in a statement.

UBS did not identify the employee, but Haig Simonian of the Financial Times reports that it is Martin Liechti, the Zurich-based head of North and South America for its international wealth-management business and one of the most senior private bankers at UBS.

The Financial Times says: "People close to the situation said the detention was an aggressive tactic and may have been chosen by the authorities to put pressure on UBS and its employees to reveal its business practices."

UBS' strength has been in its wealth-management business, providing the ultarich of the U.S., Europe, and the Middle East with expertise in investing and tax advice. The apparent escalation of tax investigations could damage that business.

German prosecutors have said they are considering an investigation into whether UBS helped wealthy Germans avoid taxes.

"UBS has been hit by a perfect storm," Edwin Merner, who oversees $2 billion at Atlantis Investment Research Corp. in Tokyo, told Bloomberg News. "Clients may run away if they think they'll leak information to tax authorities in Germany and the U.S."

Questions about UBS' possible role in tax evasion have been around for a while.

A 2003 report by the Senate Permanent Subcommittee on Investigations identified UBS as one of several major banks that had roles in questionable tax shelters set up by the accounting firm KPMG, which reached a deferred-prosecution agreement in 2005 to avoid an indictment. UBS noted in a filing with the Securities and Exchange Commission on Tuesday that federal prosecutors in Manhattan are continuing to examine UBS' conduct in some KPMG transactions during 1996 to 2000.

In the cross-border tax examination, the Justice Department, according to UBS, is looking at whether Swiss-based UBS advisers helping U.S. investors failed to register with the Securities and Exchange Commission and whether the bank violated the terms of a 2001 agreement with the Internal Revenue Service.