The US Justice Department is investigating allegations that an employee of HSBC Holdings leaked confidential client information to a major hedge fund, according to people familiar with the matter, a new twist in a cross-border government probe into misconduct in the foreign-exchange market.

US prosecutors are interviewing HSBC traders about the alleged event, which is believed to have taken place in March 2010, as part of their broader criminal investigation into improprieties in the currencies markets, these people say.