Carlyle Group LP, a private equity firm headquartered in Washington, D.C., has reached an agreement to acquire L.A. asset manager TCW Group Inc., the two sides announced Thursday.

The long-rumored deal ends more than a decade of ownership of TCW by French bank Societe Generale SA, which has been unloading assets in a bid to strengthen its capital position.

Terms of the deal were not announced, but it is structured as a private equity-led management buyout. After the sale closes, expected in the first quarter of 2013, TCW employees will own roughly 40 percent of the firm, up from about 17 percent, with Carlyle owning a majority share.

“This major milestone in the evolution of TCW enhances our long-term position in the competitive asset management business,” said Marc Stern, TCW’s vice chairman. “I am particularly pleased that we will materially increase employee ownership in TCW – a transformative change that will even more closely align our interests with those of our clients.”

TCW also announced that David Lippman, group managing director and head of fixed income, has assumed the chief executive title, effective immediately. He replaces Stern, who will become chairman once the deal closes. TCW founder Robert Day Jr. will relinquish his role as chairman, but remain as chairman of subsidiary Trust Co. of the West.

Founded in 1971, TCW has become one of the largest asset managers in Los Angeles, with more than $131 billion under management.