(Reuters) - Credit Suisse may be facing yet another investigation into its role in helping wealthy Americans avoid paying taxes after New York state's top financial regulator requested documents from the Swiss bank.

Switzerland's second-largest lender had raised expectations that it was getting past the long-running American tax controversy when it set aside an extra half a billion dollars last week to deal with a U.S. Department of Justice probe into its involvement in offshore tax evasion.

But Benjamin Lawsky, New York's financial services superintendent, is now examining whether the bank lied to New York authorities about creating tax shelters, raising the prospect of a new probe, a source familiar with the matter told Reuters.

Shares in Credit Suisse dropped 2.2 percent to 28.82 Swiss francs in Zurich on Monday as investors digested the possibility of a costly investigation.

"There is still a lot of uncertainty around all these legal issues at Credit Suisse. Nobody can tell how much it will really cost in the end," said Peter Stenz, portfolio manager of Swiss equities at Swisscanto, one of the 50 largest stakeholders in Credit Suisse.

Credit Suisse has so far set aside 895 million francs ($1 billion) to deal with tax and securities law matters in the United States, above the $780 million Swiss rival UBS paid in 2009 to settle charges it sheltered U.S. citizens from the taxman.