Manhattan's chief prosecutor has executed a search warrant as part of an investigation into food group Parmalat's operations in the United States, a person familiar with the matter said on Tuesday.
The office of New York District Attorney Robert Morgenthau could issue indictments if crimes are found to have occurred in the United States, the source said.
Morgenthau's office is "sharing information" with the US Department of Justice but is not participating in a joint investigation with the DOJ, the source said.
It was not known where the search was conducted.
Last month a spokeswoman for Morgenthau said his office was looking into the Parmalat affair. The office had no further comment on Tuesday.
Parmalat was Italy's No 8 industrial group until it plunged into insolvency in December with a multibillion-euro hole in its accounts.
Prosecutors have been trying to establish whether banks that have helped Parmalat sell some 8 billion euros in bonds since 1995 knew about the precarious state of the company's finances.
The news agencies ANSA and AGI reported on Tuesday that Bank of America, Citigroup Inc, Morgan Stanley, Deutsche Bank and UBS had been formally included in the probe of the insolvent food group's affairs.
Italy's Banca Popolare di Lodi and Banca Intesa's fund management unit Nextra were also named, they said. Several of the banks have their headquarters in New York.
A UBS spokesman said the bank was not aware it was under formal investigation. Citigroup, Morgan Stanley and Bank of America declined to comment. No one at Popolare Lodi, Banca Intesa or Deutsche Bank was available.
A senior inspector from the US Securities and Exchange Commission said in January that the way banks sold billions of euros (dollars) of Parmalat bonds was being examined.
"We need to understand if they acted in a way that was negligent or reckless or otherwise," Lawrence West, associate director of enforcement at the SEC, told the Italian newspaper Corriere della Sera.
Italian tax police have searched or taken documents from the offices of 10 companies - including the seven now reported to be formally included in the probe - as well as Parmalat.