NEW YORK – Stocks plunged in the final minutes of trading Thursday, sending the Dow Jones industrials down more than 600 points to their lowest level in five years after a major credit ratings agency said it was considering cutting its rating on General Motors Corp. The Standard & Poor’s 500 index fell more than 6 percent.

The selloff came as Standard & Poor’s Ratings Services put GM and its finance affiliate GMAC LLC under review to see if its rating should be cut. GM has been struggling with weak car sales in North America.

The action means there is a 50 percent chance that S&P; will lower GM’s and GMAC’s ratings in the next three months.

General Motors Corp. led the Dow lower, falling about 30 percent.

The Dow fell more than 660 points, or 7.2 percent, to about 8,586 in late dealings. The blue chips hadn’t fallen below the 9,000 level since Aug. 6, 2003.

The Standard & Poor’s 500 index fell 74.73, or 7.6 percent, to 910.12 in late dealings.