Prosecutor Sean Berkowitz noted an August 2001 whistleblower memo from account Sherron Watkins warning Enron founder Ken Lay that the company could implode in an accounting scandal.
Skilling, who left Enron around the same time as the Watkins memo, has testified that he did not know of the memo until months later. However, Skilling's name is a prominent feature of the Watkins memo to Lay.
Berkowitz elicited numerous "I don't know" answers from Skilling when pressed on his claim of ignorance about the memo and the likelihood of an investigation into Enron's shaky accounting.
The prosecutor then grilled Skilling on his sale of 500,000 shares of Enron stock on the first day the financial markets opened after the Sept. 11, 2001, terrorist attacks.
"You were aware of this internal investigation, you were aware there were write-downs coming, you were aware there was a lot of bad news coming," Berkowitz said. "You wanted to use the Sept. 11 tragedy" as a reason for the sale.