Attorney General Eric Holder has confirmed that the U.S. attorney's office in the southern district of New York is conducting a criminal investigation of Wall Street.

At a news conference, Holder declined to say whether the investigation involves hedge funds and allegations of insider trading — as news reports have suggested. Holder would say only the probe by the U.S. attorney's office is very serious and is ongoing.

A week ago, the FBI searched the offices of three hedge funds in New York, Connecticut and Massachusetts as part of what outside experts say could turn out to be one of the largest probes in Wall Street history. Last Tuesday, mutual fund company Janus Capital Group said it was cooperating with an inquiry on insider trading.