Allegations of corruption have tagged the Mexican Army since there was a Mexican Army-- and they aren't just allegations. The institution of "mordida" runs deep in Mexico's security organizations. When the Mexican Revolution ended many of the jefes leading militias became Army generals, but their units continued to be little more than personal instruments for local control and corruption.
When he first came into office President Vicente Fox wanted to reform the military and police. He made some inroads. The problem is that the illegal drug business has bought many police officers and perhaps even military units. For several years the officers in the US Border Patrol have complained that many of the narcotics gangs along the border have far heavier weapons than US police forces in the area. Occasionally rumors cropped up, of Mexican Army troops cooperating with the gangs (particularly around Nuevo Laredo).
This is why reports of men in Mexican Army uniforms on the US side of the border aren't a total surprise. Photographs of an incident on January 25, near Hudspeth, Texas (southeast of El Paso) show a US-military type Humvee and SUVs attempting to smuggle marijuana across the border. One SUV got stuck in the Rio Grande and the smugglers set it on fire. The Mexican government contends that the smugglers were not part of the Mexican Army. Actually, they could well be members of the Mexican Army, just not on an official operation. They could also be narcotrafficantes. Mexico forbids the possession of military-type weapons, but the drug gangs have the cash to buy any equipment they want. The Mexican government now says its soldiers will not come within 5.1 kilometers of the border "without proper authorization." This could mean several things. One thing it should mean is a "heads up" phone call to US police and military headquarters across the border in areas where Mexican troops will operate.
The Mexican government is both outraged and embarrassed. But it's an open secret that drug gangs wield great influence with its police forces.