The two agents were working in Mexico and were traveling outside Mexico City when they were stopped by gunmen at an unauthorized highway checkpoint:

The police in San Luis Potosí State said the attack occurred on a main highway about midway between Mexico City, the capital, and Monterrey, the same highway where, a month ago, a group of armed men clashed with the federal police in a running battle that left five presumed criminals dead…

American and Mexican authorities have warned that roads throughout the area are rife with false checkpoints run by drug cartel gunmen, and recent victims have not been limited to rivals. This week, a senior police intelligence official was found dead in his burning armored car in Monterrey.

On Jan. 26, two American missionaries — Sam and Nancy Davis — were ambushed on a highway between Reynosa and Monterrey. Ms. Davis was killed.

It’s not just the scale of the killing going on in Mexico that is worrisome, it’s the acceleration of the problem:

Mexico registered 15,273 gangland killings in 2010, a 58 percent increase over the previous year, and estimates the number of drug-war deaths since Calderon took office in December 2006 at more than 34,000.

We’ve been fighting a shooting war in Iraq since 2003 and the death toll there is just over three times that amount.