The reason, analysts say, is that U.S. debt is more widely held and traded than any other IOU.

A mixed slate of reports Thursday showed the economy is being held back by high gas prices and sluggish hiring.

Economists are forecasting a pickup in growth in the second half of the year. But the latest data revealed only faint signs of a turnaround.

The Commerce Department said retail sales ticked up only 0.1 percent last month, after declining in May. Consumers spent more on cars and in big chain stores in June, but less on furniture and appliances.