Benjamin Lawsky, New York's superintendent of financial services, suspects Credit Suisse officials in New York of "misrepresentations" to state regulators over its role in enabling US citizens to evade taxes, the person said.
Lawsky has demanded and obtained documents from the New York office of the Swiss bank, the source added.
The New York regulator has also sought original documents obtained by a US Senate committee that concluded in a lengthy February report that the second largest Swiss bank held assets from more than 22,000 US customers worth as much as $12 billion.
The assets were not disclosed to US tax authorities.
Lawsky "will act quickly," with action to come "within the next weeks," said the source.
Credit Suisse declined comment.
At a Capitol Hill hearing in February, Credit Suisse chief executive Brady Dougan apologized and told the Senate panel that the wrongdoing was the fault of a small group of rogue employees.
The New York investigation comes as a US criminal probe of Credit Suisse's actions to enable tax evasion nears its conclusion.
The big bank is expected to face a fine higher than the $780 million paid in 2009 by fellow Swiss bank UBS in a similar case, the New York Times reported.
However, US prosecutors are pushing for a guilty plea from a Credit Suisse subsidiary, the Times said.
Credit Suisse last week disclosed that it set aside 425 million Swiss francs, or about $476 million, due to the US Justice probe.