Chemical Missing From Mexican Truck Thu May 16,10:51 PM ET MEXICO CITY (AP) - Mexican police found a stolen truck on Thursday that had been carrying 10 tons of sodium cyanide, but most of the deadly chemicals were missing. The Environmental Ministry urged border guards to take precautions to ensure that large amounts of the chemical weren't smuggled out of the country. Officials called the alert "precautionary" and said there was no evidence any sodium cyanide had left Mexico. Authorities cordoned off the area around where the truck was discovered, abandoned along a highway in Zacatlan, about 120 miles northwest of Mexico City in central Puebla state. Only about half a ton of the cyanide, contained in barrels, was still inside the truck and investigators were searching for the remainder, police said. One barrel had been opened, sparking concerns of a health hazard. But officials found no evidence of contamination, said Frederico Perez, director of police in neighboring Hidalgo state. No one has been arrested in connection with the truck robbery, which took place last Friday. Police were seeking information about three armed men who authorities believe stole the vehicle based on witness accounts, Perez said. Sodium cyanide is used in gold and silver mining. If inhaled or ingested, it attacks the nervous system and can cause a person to suffocate within minutes. Perez said it was extremely unlikely the chemical was stolen for a terrorist attack. "The possibility of terrorism is minimal," he said. "Knowing how these robberies work, it's likely that the robbers didn't even know what was inside. They also probably wouldn't know how to use those chemicals." Truck robberies are common in Mexico, particularly in the region just north of the capital, Mexico City. The truck had left from Queretaro, a colonial city 115 miles north of Mexico City, and was headed to Pachuca, capital of Hidalgo state, about 30 miles northeast of the capital.