Stocks ended lower on Monday, as concerns about Europe returned to the forefront after major credit ratings agencies warned that European leaders had not done enough to tackle the region's debt crisis.

The Dow Jones industrial average <.DJI> was down 161.81 points, or 1.33 percent, at 12,022.45. The Standard & Poor's 500 Index <.SPX> was down 18.62 points, or 1.48 percent, at 1,236.57. The Nasdaq Composite Index <.IXIC> was down 34.59 points, or 1.31 percent, at 2,612.26.

(Reporting by Caroline Valetkevitch; Editing by Leslie Adler)