The US Securities and Exchange Commission said Monday its enforcement chief has quit, following criticism over apparent failure to uncover the alleged $50 billion investment scam run by mastermind Bernard Madoff.

SEC enforcement director Linda Chatman Thomsen planned to return to the private sector, the commission said, praising her three year stint as "a historic period."

While the commission said more than 2,000 enforcement actions were taken and billions of dollars returned to harmed investors during her tenure, some lawmakers and experts said few large busts took place under her watch.

US lawmakers last week gave the commission a severe tongue-lashing for not moving swiftly to uncover the Madoff scandal despite ample warnings, including by a private securities sleuth who said he tried for nearly a decade to sound the alarm on Madoff.

Madoff faces criminal charges in connection with the scheme that lost some $50 billion in a years-long pyramid fraud. He has been charged but not been indicted and there is no immediate indication of when he could go to trial.

New SEC chair Mary Shapiro has vowed to revamp the commission's enforcement efforts following the Madoff case, which erupted in December.

But Schapiro said in a statement Monday that Linda's achievements "have been nothing short of extraordinary, even heroic, in an era of unprecedented challenges in our securities markets."

Thomsen, who served a total 14 years at the commission, said she found her experience to have been "an extraordinary privilege."

It is not known who will replace her but it was reported over the weekend that former Assistant US Attorney Robert Khuzami, currently the general counsel for the Americas at Deutsche Bank, was a top candidate.