U.S. authorities are preparing to charge former Credit Suisse Group AG employees with criminal and civil fraud related to write-downs on subprime mortgage derivatives at the height of the financial crisis, sources familiar with the matter said.

Credit Suisse will not be charged in the matter, which is being investigated by federal prosecutors and the U.S. Securities and Exchange Commission, the sources said.

The investigation stems from $2.85 billion in write-downs that Credit Suisse took on collateralized debt obligations in 2008, said the sources, who spoke on the condition of anonymity.

Credit Suisse revealed those CDO losses in early 2008, and blamed them on a group of rogue traders whom the bank said had deliberately mispriced securities, and a failure of internal controls.

Spokespeople for Credit Suisse, the Federal Bureau of Investigation, the SEC and Manhattan U.S. attorney Preet Bharara declined to comment on the matter.

Charges could come as early as Wednesday, people familiar with the matter said, but the timing was uncertain.

The planned charges come as the Obama administration is stepping up efforts to prosecutes Wall Street bankers and others for misconduct related to the financial crisis. Last week during his State of the Union address, President Obama announced the formation of a mortgage-fraud task force to pursue subprime-related cases.

The collapse of the subprime housing market was one of the triggers of the worst financial crisis since the Great Depression.

(Additional reporting by Matthew Goldstein, Basil Katz and Sarah Lynch)