The Department of Justice has opened a criminal investigation into whether some HSBC Holdings Plc clients may have failed to disclose offshore accounts, lawyers familiar with the probe said on Tuesday.

The London-based bank has become the latest focus of a widening federal probe into U.S. citizens believed to have stored billions of dollars of assets in offshore accounts to avoid scrutiny by domestic tax authorities.

HSBC clients have received a June 21 letter from Kevin Downing, a senior litigator at the Justice Department who oversaw the UBS probe, that says they are subjects of a criminal investigation. Reuters obtained a copy of the letter.

A Justice Department representative did not immediately return an email request for comment.

Last year, Switzerland's UBS AG admitted to criminal wrongdoing for helping U.S. clients evade taxes. It agreed to pay a $780 million penalty and hand over details on more than 250 client accounts, and later agreed to disclose names of 4,450 wealthy U.S. clients to tax investigators.