Documents reportedly obtained by Bloomberg News show the non-government Financial Industry Regulatory Authority (Finra) has written to an unspecified number of the 5,000 brokerages it oversees requesting lists of all the clients they funneled to Mr Madoff's firm since 2005.
It has also asked for accounts of fees generated by these referrals, materials used to promote Mr Madoff's firm and company files concerning the due diligence carried out on the fund manager's business.
Mr Madoff is currently awaiting trial for securities fraud over an alleged $50 billion Ponzi scheme.
According to Bloomberg's figures, investors including hedge funds, banks and private individuals have so far disclosed losses of $41 billion from the scandal.
In related news, interim Finra chief executive Stephen Luparello was scheduled to testify before the Senate bank committee yesterday (January 27th) as part of its hearings into regulators' oversight of Mr Madoff and their apparent failure to detect the alleged fraud.
His predecessor, new SEC chairman Mary Schapiro, had previously said Finra did not have the authority to investigate the broker's activities.