Officials in Puebla told reporters that two of the barrels were found open, and that some barrels were still missing. Police said experts would examine the recovered barrels to make sure they still contained the chemical.

A truck carrying 10 tonnes of the industrial chemical in 96 barrels was stolen on May 10 as it traveled from the central Mexican city of Queretaro, Queretaro, to Pachuca, Hidalgo.

Robberies of commercial trucks are common on Mexico's highways, and the shipments are usually sold through fences.

The truck was found abandoned on May 16 with part of its shipment. Officials never said how many barrels were on the truck when it was recovered, but local press said 20.

U.S. Customs officials issued a series of alerts about the stolen truck on the U.S.-Mexico border and scrutinized vehicles for the cyanide, on concerns it could be put into water supplies or otherwise used in a terrorist attack.

Mexican authorities said from the beginning they expected the thieves probably stole the truck and cargo to sell them and were oblivious to what was inside the drums.

Sodium cyanide is used to separate gold and silver from the ore in which they are found, to harden steel, and to produce fabrics, rubber and pharmaceuticals.