(CNN) -- They're known as "monsters" and they rumble in northeastern Mexico as such.
Mexican officials say the custom-made armored trucks are used by drug traffickers to transport drugs headed for the United States and weapons back to Mexico. The bulletproof tanks also are used as weapons of war in clashes between rival drug cartels.
These narco-tanks are designed to serve several purposes. Built on a three-axle truck bed, they're fitted with swiveling turrets to shoot in any direction. A reinforced steel battering ram is welded to the front of the vehicle to destroy vehicles and demolish walls. One-inch-thick steel plating is meant to protect occupants from deadly gunfire and grenade blasts.
Two of these armored trucks were seized by the Mexican army in Tamaulipas, a Gulf Coast state just south of Texas. Two more were in the process of being built and 23 big-rig trucks were apparently in line for future "monster" projects.
A spokesman with the Mexican military who chose not to be identified for security reasons said that the trucks were seized as part of an operation against drug traffickers. The raiding soldiers "caught by surprise several armed civilians who were running into a clandestine workshop where they were indeed making this kind of vehicles used by the cartels to transport drugs headed for northern Mexico," said the spokesman.
The clandestine workshop was in the town of Camargo, he said. Inside, authorities found the other trucks as well as multiple steel plates and machinery allegedly used to build the armored trucks.
A look inside these "monster" trucks is very revealing. They have hatches and peepholes for snipers. Their spacious interiors can fit as many as 20 armed men and they are coated with polyurethane for insulation and to reduce noise. They also have air conditioner ducts as well as benches and bars for occupants to hold onto.
The inch-thick steel plating, according to the military spokesman, can withstand weapons up to 50 calibers and grenade explosions.
The military spokesman also said that "to destroy this kind of vehicle you would have to use anti-tank weapons."
So far Mexican security forces have seized 20 narco-tanks in various locations in northeastern Mexico, all with very similar features, the spokesman said. They suspect there may be numerous other clandestine workshops where these armored trucks are still being made.
Mario Gonzalez contributed to this report