The FBI put out an alert to law enforcement agencies Wednesday to be on the lookout for a tractor-trailer combo, hijacked in Mexico last week, that bears a cargo of 96 heavy-duty drums of the rock form of the industrial chemical sodium cyanide. The total haul is 21,120 pounds of the chemical.

The truck was commandeered by three hijackers armed with handguns last Friday in the Mexican state of Hidalgo, some 500 miles south of Brownsville, Texas. Mexican federal authorities alerted U.S. law enforcement to the situation on Monday, due to the volatility of the truck's cargo, and the FBI then joined the search for the truck.

A U.S. law enforcement source told Fox News that there is no reason to believe that the truck has crossed the border into the United States, and the border patrol has been on the lookout for the vehicle since Mexican authorities reported it missing.

The FBI bulletin says the truck was transporting the drums of sodium cyanide from a distribution center in Queretaro City to a mining customer near the town of Ixmiquilpan, Hidalgo, when the driver was forced from the vehicle by the three armed subjects.

"When mixed with any type of acid, sodium cyanide produces a lethal toxic gas that would have disastrous impact on a broad geographic area," the FBI alert said. "If sodium cyanide catches fire it produces toxic gases that are difficult to extinguish."

"Firefighters are trained to use dry chemicals and sand rather than water to extinguish the flames," the alert continued. "Standard municipal water treatment processes should filter sodium cyanide out of contaminated source waters, however, the introduction of sodium cyanide into untreated water would be poisonous to humans and wildlife."

Some of sodium cyanide's legitimate uses include use in mining processes that extract gold or silver from raw ore. In smaller amounts, the chemical compound can also be used in electroplating chrome for cars.

"Due to the potential health risks posed by the truck's cargo, law enforcement officers should exercise extreme caution when approaching suspicious vehicles that appear to match the description provided," the alert said. "As a precautionary measure, officers should contact hazardous materials response units to respond to sites that may contain sodium cyanide."