MONTERREY, Mexico — Soldiers on patrol in a Mexican border town discovered a warehouse where armor-plated “tanks” were being prepared for the Gulf drug cartel, a military source said Monday.

The patrol came across the warehouse when they clashed with a group of armed men in the town of Ciudad Camargo, in the far northeastern state of Tamaulipas. Two of the gunmen were killed in a firefight, while two hid inside the warehouse.

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“We found two home-made armored trucks in the warehouse, which belongs to the Gulf Cartel,” the military source told AFP, speaking on condition of anonymity.

The trucks were covered in steel plates one inch (2.5 centimeters) thick, strong enough to “resist the caliber of personal weapons the soldiers use,” said the source.

The air-conditioned armored vehicles were equipped with portholes where snipers could open fire from and remain protected.

Soldiers also found two other trucks that were in the process of being armored, as well as 23 powerful big-rig trucks that were apparently going to be armored.

The vehicles, locally known as “monsters,” can even resist fire from a heavy .50 caliber machine gun and can only be destroyed with anti-tank weapons, according to the military.

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The home-made tanks are used in clashes with other drug cartels as well as to protect drug shipments.

In recent years, soldiers deployed in the northeastern Mexican border region have confiscated 109 home-made armored vehicles — including one dubbed the “Popemobile” because it carried an armored cabin similar to that used to protect Pope Benedict XVI in foreign trips.

In May, police in the western state of Jalisco carrying out a sweep against the Los Zetas drug cartel discovered an armored vehicle large enough to carry 20 armed men and also equipped with weapons portholes.

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Members of the Gulf Cartel and the Zetas are engaged in a bitter fight to control the lucrative smuggling routes in eastern Mexico into the United States.

Separately, investigators in the northern state of Coahuila said Monday that they had discovered 1,314 pieces of human bones that they believe are the remains of victims whose bodies were burned.

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Soldiers found the remains in 20 pits near the town of Guerrero along with 60 bullet shells and personal items such as clothing and watches.

Mexico has seen an explosion in drug-related violence which has left some 37,000 dead, according to media reports, since the government launched a military crackdown on organized crime in 2006.