It may have been written off as a hopeless case less than a year ago, but the stock of the American International Group shot up to $50 on Thursday, capping a fourfold gain in the last two months.

For all the optimism taking hold in the markets these days, it is hard to find a tangible explanation.

"Who would want to buy a stock that's still 80 percent owned by the government?" wondered William T. Fitzpatrick, an equity analyst at Optique Capital. Shares ended the day at $47.84, a gain of 27 percent from the previous close of $37.69.

Yes, the company has named a new chief executive, who comes from a solid background at MetLife, and yes, he has said that A.I.G. will pay back the government for its bailout sooner rather than later. The giant insurer has been moving to change the names of key business units and working to disentangle its bewildering structure. It even reported a profit in its latest quarter.

But none of that really explains the recent gains. Speculation swirls daily that some deal may be in the works, that short sellers are being squeezed out of their positions, or that the company's former chief executive, Maurice R. Greenberg, may be poised to make a comeback in the role of consultant. The new chief executive, Robert Benmosche, has been fueling some of the interest, as he talks about seeking