By Lynnley Browning

(Reuters) - A top prosecutor responsible for a crackdown on Swiss banks and sellers of corporate tax shelters has resigned from the Justice Department, a source familiar with the matter said on Thursday.

Kevin Downing, 46, who has led agency criminal probes of the Swiss banking industry, and earlier of banks and accounting firms, is leaving to join a law firm, the source said.

The name of the law firm could not be immediately determined.

His departure comes amid intense scrutiny of Swiss banks by the Justice Department and the Internal Revenue Service.

Eleven banks, including Credit Suisse AG, are under criminal investigation. Wegelin & Co, Switzerland's oldest private bank, was indicted in February.

Dozens of Swiss private bankers and their American clients have been indicted in recent years.

Downing joined the Justice Department in 1997. He led the investigation of banks, accounting firms and law firms that sold bogus tax shelters to retail investors.

Under his direction, a probe culminated in a deferred-prosecution agreement with KPMG in 2005. The Big Four accounting firm averted indictment by admitting to criminal wrongdoing and paying a $456 million fine.

Downing also led the investigation of UBS AG, Switzerland's largest bank. In 2005, UBS entered into a deferred prosecution agreement and paid a $780 million fine over tax-evasion services sold through its private bank to wealthy Americans.

Charles Miller, a Justice Department spokesman, declined Thursday to comment on Downing's departure.

(Reporting by Lynnley Browning; editing by Kevin Drawbaugh and Jeffrey Benkoe)