Investigators probing the alleged fraud carried out by Bernard Madoff are looking at a key lieutenant at the firm and have issued a subpoena to the accountant who audited the firm's financial statements, the

Wall Street Journal

reports, citing people familiar with the matter said.

Authorities are trying to determine who helped Madoff carry out what they say appears to be at least a 30-year scheme that may have caused at least $50 billion in losses. They are seeking information from the accounting firm that handled Madoff's audits and are examining the role of Frank DiPascali, who dealt with client accounts and worked at Madoff's firm for more than 30 years, the

Journal

reports.

After Madoff's arrest on Dec. 11, investigators from the

Securities and Exchange Commission

showed up at the Madoff firm's headquarters in Manhattan and questioned DiPascali. He told the SEC he didn't know who was responsible for clearing and settling trades in the investment-advisory side of the firm, according to an SEC memorandum reviewed by the

Journal

.

DiPascali hasn't been charged with any wrongdoing.

The

Journal

also reports investigators issued a subpoena to David Friehling, a New City, N.Y., accountant who audited the Madoff firm's financial statements, and are seeking documents related to the Madoff firm going back to Jan. 1, 2000. He has until Dec. 29 to fulfill the request. Andrew Lankler, a lawyer for Friehling, declined to comment, the newspaper reports.

This article was written by a staff member of TheStreet.com.