NEW YORK (Bloomberg) — Billionaire Kirk Kerkorian may sell his stake in Ford Motor Co. after the $1 billion he invested lost two- thirds of its value and put his firm's controlling interest in casino company MGM Mirage at risk.

Kerkorian's Tracinda Corp., which paid as much as $8.50 a share for Ford, sold 7.3 million shares on Monday for an average of $2.43 each and has contacted an investment bank about unloading the rest, the company said in a regulatory filing yesterday. Tracinda still has 133.5 million shares left, and based on Monday's closing price, they were valued at $311.1 million.

Kerkorian's bet on the second-largest US automaker have been tied to his stake in MGM Mirage because he pledged the casino's shares as backing on a $600 million credit line he used to buy Ford's stock. The 91-year-old investor offered his stake in MGM as a guarantee against the falling value of Ford even as the credit freeze threatened to send US auto sales to their lowest levels since 1991.

"It was an investment that made no sense," said Maryann Keller, an independent auto analyst and consultant in Greenwich, Connecticut. "He's pulling in his horns and concentrating on areas he knows best."

Tracinda's filing said it "intends to further reduce its holdings" in Ford to concentrate on gambling, hotels and energy. It didn't give specifics.

Mark Truby, a spokesman for Dearborn, Michigan-based Ford, said the company would "stay focused" on its turnaround plan. "Any questions about Tracinda's investment should be directed to Tracinda," he said.

Kerkorian disclosed in late April he had acquired 100 million shares of Ford, and said June 19 he had boosted his stake to 140.8 million shares, or 6.43 percent of the company. He expressed support for chief executive officer Alan Mulally's efforts to revamp the automaker after $23.9 billion in losses since 2005.

Kerkorian's sale came five days after Ford's collapsing stock price forced him to pledge another 50 million shares of MGM Mirage to support the credit line. Ford's shares are down 65 percent for the year.

Winnie Lerner, an outside spokeswoman for Tracinda with Abernathy MacGregor Group, declined to comment about Tracinda's investments. Tracinda is the majority owner of Las Vegas-based MGM, the world's second-largest casino company, and holds a 35-percent stake in Denver-based Delta Petroleum Corp., according to Bloomberg data.