The U.S. government said Monday it so far cannot explain a reported incident of sudden, high-speed acceleration in a Toyota Prius on a San Diego, Calif., freeway last week.

Concerns about China's economy and a warning about the U.S. credit rating sent stocks modestly lower.

The National Association of Home Builders says its housing market index slipped in March, as harsh winter weather and competition from foreclosed properties hurt builders' sales prospects.