Bill Ackman said Hindenburg Research has “outed” the way billionaire Carl Icahn runs his publicly traded company and suggested shares have room to fall after tumbling to the lowest levels since 2009.

In a lengthy Twitter post, Ackman, 57, also called out 87-year-old Icahn’s use of margin loans against his shares in Icahn Enterprises LP. The stock plunged more than 13% on Wednesday, with the price reaching the lowest level in more than 14 years. Ackman, who has clashed with Icahn in the past, said his firm was neither long nor short — “just watching from a distance.”

“$IEP reminds me somewhat of Archegos where the swap counterparties were comforted by each having relatively smaller exposures to the situation,” Ackman said, referring to Bill Hwang’s family office that spectacularly blew up in 2021.

“All it takes is for one lender to break ranks and liquidate shares or attempt to hedge, before the house comes falling down,” Ackman said. “Here, the patsy is the last lender to liquidate.”

Neither Icahn nor Ackman immediately responded to a request for comment.

Icahn and Ackman have plenty of history. The two were on opposite sides of a five-year battle over Herbalife Ltd., with Icahn buying into the stock in 2013, months after Ackman took a short position in the company and labeled it a pyramid scheme.

The two publicly faced off over the bets multiple times, with Icahn assailing Ackman on stages, television, in documentaries and online. Icahn won out as the stock climbed and Ackman eventually sold out of his short position.

Hindenburg, run by Nate Anderson, put out a report earlier this month that argued Icahn’s stock was significantly overvalued relative to its underlying holdings, and that the company’s high dividends aren’t sustainable. It didn’t allege wrongdoing.

“We have full confidence in the integrity of our presented financials and our reporting,” David Willetts, chief executive officer of Icahn Enterprises, said during an earnings call about a week after the report. The company is “well-positioned for future success.”

In a 90-minute interview with Bloomberg News on May 21, Icahn declined to directly address Hindenburg and its report, which his company has called “self-serving.” He said he was focused on his latest target, Illumina Inc., and had recently discussed the possibilities of artificial intelligence with his son, Brett.

“People come and ask me, ‘How do you feel?’ Maybe it sounds strange, but it doesn’t really affect me a whole lot. It’s my nature,” Icahn said in the interview.

Icahn owns about 84% of Icahn Enterprises. He has an estimated net worth of $8.9 billion, according to the Bloomberg Billionaires Index. Ackman is worth about $2.5 billion, according to the index.

“Over his storied career, Icahn has made many enemies. I don’t know that he has any real friends,” Ackman said in his tweet. “He could use one here.”

(Adds details on Ackman and Icahn’s history starting in sixth paragraph.)