Phillip Bennett, the former chief executive of Refco, and others have been indicted in connection with the Refco collapse.

"Acting hand-in-hand with Bennett, Collins made affirmative misrepresentations, material omissions, and told deceptive half-truths, all to assist Bennett's scheme to steal more than $2.4 billion from potential investors," the indictment said.

Lawyers are rarely charged criminally in connection with a client's alleged fraud. In the collapse of Enron Corp., no outside lawyers were charged. "There tends to be gray in legal transactions, but to show intent in a white collar prosecution, it needs to be black and white," says Andrew Weissmann, the former head of the Department of Justice's Enron Task Force and now a partner at Jenner & Block LLP. "It's difficult to develop that kind of evidence against lawyers."

The Securities and Exchange Commission also filed a civil complaint Tuesday against Mr. Collins, alleging that he aided and abetted securities fraud at Refco.

Mr. Collins, who has been the head of Mayer Brown's derivatives group, is now on leave from the firm while the charges are pending, the firm said Tuesday. "Mayer Brown has cooperated fully with authorities investigating activities related to the collapse of Refco," the firm said in a statement. "Our review of the evidence available to us shows that the firm acted in a professional, competent and ethical manner in its work on behalf of Refco."

Mr. Collins's lawyer, William Schwartz, said his client intended to fight the charges and called him "an innocent victim of the Refco fraud. This indictment should send a chill down the spine of every transactional lawyer who believes he or she is representing an honest client."

Michael Garcia, the U.S. Attorney for the Southern District of New York, took a different view, saying it is "not a crime to have a client who commits a crime. No lawyer will be prosecuted unless that lawyer knows about the client's fraud and agrees to join in it understanding its unlawful nature."

The charges are a blow to a law firm that has weathered several of late. In July, buyout firm Thomas H. Lee Partners LP, which purchased a stake in Refco in 2004, sued Mayer Brown for allegedly failing to inform Lee about the bogus loan transactions. Also this year, an independent examiner filed a report in Refco's bankruptcy, concluding that there was significant evidence that the law firm "knew or should have known" the loans were fraudulent.

Beyond Refco, the law firm has struggled with recent defections by high-profile partners, and it suffered negative publicity this year when it fired or demoted 45 partners in an effort to boost its profitability.