NEW YORK (CNN/Money) - Ex-ImClone CEO Samuel Waksal pleaded guilty Tuesday to securities fraud charges related to an insider trading scandal that could envelop home design guru Martha Stewart.

In an unusual move, prosecutors also said they are investigating a previously undisclosed sale of $30 million in ImClone stock by a Waksal associate that may result in new insider trading charges.

Waksal's guilty plea in U.S. District Court was the second in the investigation of insider trading of ImClone stock. An assistant to Stewart's stock broker pleaded guilty this month to a misdemeanor charge and agreed to testify against people charged in the case.

"Sam Waksal's guilty plea has nothing to do with Martha Stewart," Stewart's lawyers said in a statement Tuesday.

Waksal did not implicate Stewart, and his plea was not part of an agreement to cooperate with prosecutors. But that does not necessarily mean Waksal won't cooperate in the Stewart case, according to some former federal prosecutors.

Former ImClone CEO Sam Waksal comments on his guilty plea. Play video (QuickTime, Real or Windows Media)

"With a plea agreement, he [Waksal] has a chance of avoiding prison if he cooperates against others," said Seth Taube, one of the former prosecutors. "He had to flip on someone, and if you're not going to flip on family, you're going to flip on your best friend. Besides, the government wanted Martha worse. They're doing it publicly and that's a message to Martha tied in a little ribbon."

At issue is whether Stewart acted on illegally obtained information in selling her stock. She has denied any wrongdoing and has not been charged. She has maintained that she had a standing order with her stockbroker, Peter Bacanovic, to sell the shares if the stock dropped below $60.

Sheila Sawyer, another former federal prosecutor now in private practice, said Waksal's decision not to plead guilty to charges related to tipping his family members at least leaves open the possibility that he might be cooperating with prosecutors in building a case against Stewart.

"Based on everything I've read, Waksal is very anxious to avoid serving a very lengthy prison sentence," said Sawyer, of the Foley Hoag firm in Boston. "One of the best ways to avoid that is to cooperate with the government in its investigation."

However, Sawyer cautioned that it's also possible Waksal has no useful information to share about Stewart.

Appearing before U.S. District Court Judge William Pauley III in federal court in Manhattan, Waksal pleaded guilty to six counts of securities fraud and obstruction of justice, though he did not plead guilty to conspiracy to tip off family members, namely his daughter and father.

The plea was not made in exchange for a predetermined sentence agreement with prosecutors -- an unusual arrangement. Prosecutors also did not agree to dismiss other charges pending against Waksal, and said they may add more serious counts.

Additionally, assistant U.S. Attorney Michael Schachter said in court Tuesday that Waksal tipped others about a pending FDA action that affected ImClone, who then sold stock on Dec. 27 and 28.

One person Schachter described as "a close friend of Dr. Waksal" sold $600,000 worth of shares on Dec. 28, and another person sold $30 million of ImClone stock on Dec. 27 or 28. Both trades are being investigated, Schachter said.

Schachter told the judge that phone records indicate a conversation between Waksal and the person just before the sale was made.

In outlining the accusations against Waksal, Schachter issued a stinging attack on the disgraced ex-CEO, charging he had used his daughter, Aliza, and his father as "human shields" to try to save himself from prosecution.

Waksal's lawyers had been trying to negotiate a plea agreement with prosecutors in hopes of protecting his daughter and father from federal insider trading charges.

"I regret more than anything having involved my daughter Aliza, who has done nothing wrong," Waksal said in a shaky voice as he read from a statement outside the courthouse. He added: "The media attention brought about because of me has cast a shadow on ImClone."

Waksal was arrested in June and indicted in August on charges of trying to sell his ImClone shares and tipping off family members who held millions of dollars in company stock a day before the Food and Drug Administration said it would not review ImClone's application for its experimental cancer drug, Erbitux.

He could face up to 65 years in prison for the charges to which he pleaded guilty. It was not immediately clear Tuesday whether prosecutors will pursue the charges he did not plead guilty to during the hearing.

Authorities are investigating whether Waksal tipped off Stewart, who sold nearly 4,000 ImClone shares Dec. 27, the day before the FDA's announcement. The Justice Department is investigating whether Stewart knowingly lied to lawmakers about her stock sale.

An assistant at Merrill Lynch to Bacanovic, Douglas Faneuil, initially backed Stewart's account, but later admitted that he was not honest when first interviewed by investigators from the Securities and Exchange Commission and FBI agents.

As part of a guilty plea entered earlier this month, Faneuil agreed to testify against Stewart and others if they are charged in connection with the scandal. Faneuil pleaded guilty to a misdemeanor charge of accepting gifts to withhold information about Stewart's sale of her shares.

Waksal is just one of many executives who have been accused of white-collar crimes this year. Since the collapse of Enron Corp. last year, former executives of WorldCom, Tyco International and Adelphia Communications, as well as Enron, have been indicated for fraud, tax evasion, and on other charges.

Waksal resigned in May, when the company learned that federal charges were going to be filed.  

-- The Associated Press contributed to this report