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The Justice Department and the New York Attorney General are among those probing Credit Suisse's actions, according to the sources, who spoke on condition of anonymity.
A spokesman for Credit Suisse, Switzerland's second-largest bank, declined to comment.
Zurich-based Credit Suisse is the second bank known to be targeted by U.S. authorities probing how banks bundled mortgage loans into securities during the U.S. housing boom.
New York Attorney General Eric Schneiderman filed a civil fraud case against JPMorgan Chase & Co on Monday over mortgage-backed securities originated and sold by Bear Stearns.
The JPMorgan complaint was the first action to come out of a working group created by President Barack Obama earlier this year to go after wrongdoing that led to the 2008 financial crisis. JPMorgan, which bought Bear Stearns for $10 a share in March 2008, said in a statement it would contest the allegations.
The nature of the investigation of Credit Suisse was not immediately known. However, private parties have filed lawsuits against the bank claiming that it misrepresented the quality of mortgages underlying securities it sold.
The U.S. Securities and Exchange Commission subpoenaed mortgage-related documents from Credit Suisse, bond insurer MBIA Inc disclosed in a court filing in May 2011.
The New York attorney general's office declined to comment. The Justice Department did not immediately return calls and emails seeking comment. An SEC spokesman declined comment.
The Residential Mortgage-Backed Securities Working Group was formed to probe the pooling and sale of risky mortgages in the run-up to the 2008 financial crisis.
Obama said he created the group to "hold accountable those who broke the law" and to "help turn the page on an era of recklessness".
(Reporting by Karen Freifeld and Alison Frankel; Editing by Eddie Evans, Bernard Orr)