An Architect of the Tourre Case to Depart S.E.C.

Kenneth R. Lench oversaw the Securities and Exchange Commission’s case against Goldman Sachs and Fabrice Tourre, a trader at the bank.

On Tuesday, a day after Mr. Tourre’s trial began, the S.E.C. announced that Mr. Lench’s 23-year career at the agency was ending. Mr. Lench, the agency said, will join a private law firm after he departs at the end of July.

Mr. Lench played a significant role in the S.E.C.’s response to the financial crisis. When the agency created the “structured and new products unit” in early 2010 to investigate mortgage securities and other complex financial instruments at the center of the crisis, it chose Mr. Lench to lead the 45-person team.

Under his control, this group filed civil enforcement actions against some of the biggest names on Wall Street, including JPMorgan and Citigroup. The most prominent case came against Goldman and Mr. Tourre, which it accused of misleading investors about a mortgage security that ultimately failed.

Goldman chose to settle the case, paying a $550 million penalty without admitting or denying guilt. Mr. Tourre fought the charges.

His civil trial continued on Tuesday in a federal courtroom in lower Manhattan. His lawyers, who have portrayed Mr. Tourre as a fall guy for a deal that was approved at higher rungs of Goldman, are expected to argue that investors were well aware of the mortgage security’s contents.

While Mr. Lench oversaw the investigation of the case, he was not involved in the trial.

Mr. Lench, who joined the S.E.C.’s enforcement unit in 1990, also led the S.E.C.’s auction rates securities settlements with several banks like UBS and Bank of America. He also investigated accounting fraud.

“It has been a privilege to lead such a talented and dedicated team of professionals committed to uncovering wrongdoing in our securities markets, Mr. Lench said in a statement. “I am fortunate to have had the opportunity to work on significant and challenging cases on behalf of our nation’s investors.”