WASHINGTON — The volume of signed contracts to buy previously occupied homes rose for the eighth straight month in September as buyers scrambled to take advantage of a tax credit for first-time owners that expires at the end of this month.

The National Association of Realtors said today its seasonally adjusted index of sales agreements rose 6.1 percent from August to 110.1. It was the highest reading since December 2006 and more than 21 percent above a year ago. Economists surveyed by Thomson Reuters expected the index would be level at 103.8.