At least six US companies have received subpoenas from federal prosecutors over investigations into suspect finite reinsurance deals.

New York Attorney-General Eliot Spitzer, who voiced reservations about the legitimacy of such transactions, initially spurred this action.

Insurance companies Assurant, XL Capital and Platinum Underwriters, reinsurers PartnerRe and Renaissance Re and auto insurer Bristol West Holdings have so far reported requests for documents from the US Attorney for the Southern District of New York, while other companies have reported receiving similar subpoenas.

Authorities are still examining whether finite reinsurance contracts can be used to cover up losses on company records.

Finite reinsurance deals have been used for years to smooth out discrepancies in company results. While they appear as reinsurance contracts on the balance sheet, they can be used as a type of loan.