Earlier this week, Credit Suisse announced that it was to pay a $536 million fine after admitting its involvement in transactions involving Iranian companies and said the practice had been stopped in 2005.
But US authorities explained that its misconduct had been far more widespread and deliberate than Credit Suisse admitted to in its statement, reports Reuters.
US Attorney General Eric Holder said: "Credit Suisse's decades-long scheme to flout the rules that govern our financial institutions robbed our system of the legitimacy that is fundamental to its success."
According to documents presented to the court, practices to help clients evade US sanctions may have begun at Credit Suisse as early as 1986, when the government barred companies from trading with Libya.
Stuart Levey, Treasury under-secretary for terrorism and financial intelligence, cited an example of the practices that had been taking place at the Swiss bank.
"Credit Suisse learned that another international bank had ceased to handle Iranian banks' US dollar clearing business," he said.
"Instead of perceiving potential risk, Credit Suisse saw a business opportunity and sought to take over the business."
In its statement, the bank said it had now changed its working procedures to ensure such practices do not happen at the company again.
Manhattan District Attorney Robert Morgenthau has said other banks are being investigated for similar offences, with transactions believed to have violated US sanctions against the likes of Cuba, Liberia and Libya.
An unnamed source told Reuters that nine banks are under suspicion and four, including Credit Suisse, have already made settlements.
Earlier this year, the UK-based Lloyds Bank paid $350 million to the US in regard to allegations that it faked records to let clients from Sudan and Iran do business in the US banking sector.
By Gary Cooper