DETROIT -- Billionaire investor Kirk Kerkorian has started his previously announced $170 million cash offer to buy up to 20 million additional shares of Ford Motor Co.

Kerkorian's investment company Tracinda Corp. said today that the offer at $8.50 per share runs until June 9, unless it is extended.

If the full purchase is made, Tracinda would own 120 million shares of the Dearborn-based automaker's common stock, or about 5.5 percent of outstanding shares.

Ford shares are down 7 cents at $8.13 in morning trading. The $8.50 price Kerkorian is offering is a 13.3 percent premium over Ford's closing price before Kerkorian announced the offer last month.

That announcement touched off speculation on how he might try to influence the automaker.