A group of billionaires, hedge-fund managers and ex-Goldman Sachs executives are building a banking empire in California — with assistance from Uncle Sam.

Exactly a year ago these investors acquired the assets of collapsed mortgage lender IndyMac Bank from the federal government for about $1.5 billion and renamed it OneWest Bank. Earlier this month, OneWest purchased First Federal Bank of California, a failed Los Angeles lender. With the purchase, Pasadena-based OneWest more than doubled its branches to 72 and increased its total assets by a third to $24 billion, making it the largest bank based in Southern California.

The OneWest ownership roster reads like an excerpt of the Forbes 400. It includes J.C. Flowers & Co., an investment firm run by former Goldman Sachs Group Inc. banker J. Christopher Flowers; Paulson & Co., the large hedge fund; MSD Capital, which invests the fortune of computer mogul Michael Dell; and a fund controlled by famed speculator George Soros.

How successful OneWest’s owners are with the venture — both financially and in the public eye — will influence whether the government welcomes more private capital into the banking system.

• Read the full Wall Street Journal story.