HOUSTON — Former Enron Corp. Chief Executive Jeffrey Skilling was among American shareholders who sold stock at their first opportunity days after the Sept. 11 terrorist attacks.

But prosecutors in his fraud and conspiracy trial allege he sold 500,000 Enron shares on Sept. 17 that year because he had inside information that the energy company was in big trouble, not because he was a panicked shareholder in a roiled market.

Yesterday, testimony turned to that specific trade, which raked in $15.5 million and is among 10 improper insider trades Skilling is alleged to have made. Skilling is on trial alongside company founder Kenneth Lay, but only Skilling faces charges of improper stock sales.

Also yesterday, Lay's legal team sought to weaken testimony from ex-Enron treasurer Ben Glisan Jr. that Lay lied to employees and Wall Street about Enron's financial health in October 2001, weeks before the company spiraled into bankruptcy.

Regarding Skilling's trade, stockbroker Glenn Ray testified that the former CEO called him on Sept. 17, 2001, and said he wanted to sell 500,000 shares.

However, Skilling had called Ray on Sept. 6 — before the attacks — and said he wanted to sell 200,000 Enron shares. That sale wasn't executed because Skilling told Ray he was no longer an officer at Enron, and therefore didn't have to report trades to the Securities and Exchange Commission. The broker wanted a letter from Enron verifying that, and Skilling said he would get one.

Skilling resigned abruptly from Enron in mid-August 2001, citing personal reasons.

Glisan and other witnesses have testified so far that Skilling and Lay knew in 2001 that the company faced multibillion-dollar writeoffs on poor assets and was propped up by accounting maneuvers rather than healthy business operations.

Nine of Skilling's allegedly illegal stock trades took place before he resigned.

Both Skilling and Lay deny any wrongdoing and attribute Enron's failure to negative publicity and diminished market confidence.

Prosecutor Leo Wise played audiotapes yesterday for jurors of phone calls Skilling made to Ray regarding the stock sale, during which Skilling told the broker he wanted to sell 500,000 Enron shares quickly because "the market is dropping now."

Skilling also told Ray on the call, "You can't do anything if you have material inside information, but that's true for any human being on the planet and that's a decision I have to make."

Skilling lawyer Ron Woods tried to show his client sold stock on that date because the stock market was plummeting on the first day it opened after the attacks.

"You were aware the market was dropping that day, every hour on the hour, correct?" Woods asked.

"It was dropping, yes," Ray replied.