Prosecutors reportedly have another witness in their investigation into whether Martha Stewart had insider information when she sold a block of ImClone Systems ( IMCL) shares shortly before a negative FDA ruling was made public.

A Connecticut real estate agent who was traveling with Stewart, Mariana Pasternak, is cooperating in the probe, although it's not clear what she has told prosecutors, The Wall Street Journal reported. Pasternak was en route to a Mexican vacation with Stewart, head of Martha Stewart Omnimedia ( MSO), on Dec. 27, the day Stewart sold her roughly 4,000 ImClone shares. Pasternak was questioned by congressional investigators last month but they were asked to "lay off" the witness when she began cooperating with prosecutors.

Sam Waksal was indicted this week on fraud charges for allegedly forwarding news of the FDA rejection to his daughter and father and trying to use it to sell his own shares. A brokerage assistant at Merrill Lynch ( MER) has reportedly told prosecutors that Stewart's sales occurred after he told her of the family members' sell orders. He is currently negotiating an immunity agreement with the government.

Stewart says she sold the shares because of a stop-loss arrangement with her Merrill broker, Peter Bacanovic, who is also the Waksals' broker and who is also under investigation. Stewart has denied any wrongdoing and her lawyer declined to comment for the Journal story.