PUEBLA, Mexico -- If it were not for its extremely deadly cargo, the truck might have been just another of many stolen on Mexico's highways.

The driver said he was ambushed after pulling over to assist another trucker. After tying up the would-be Samaritan, three thieves drove off with his new semi-trailer and its cargo of 96 tons of sodium cyanide, one whiff of which can kill a person in minutes.

For the last two weeks, the mystery regarding the stolen load of mining-industry chemicals has stumped authorities. A widespread hunt has spanned half a dozen central Mexican states while police, health officials and border guards have been put on alert because of the danger.

Officials have downplayed concerns that the thieves could have terrorists or that the fatal substance might be headed for the United States. But they have not been able to rule out any motive.

"Nobody has called to give us clues, true or false. We are very amazed and worried," said Guillermo Melgarejo Palafox, director of civil protection for the state of Puebla, where the truck was later found, minus most of the chemicals.

"It's not possible to say it would never be used" for criminal purposes, he said. "It's impossible to know everyone's intentions, but it's more likely that it will be sold on the black market for other ends."

Officials at the U.S. Embassy in Mexico City have declined to say whether the missing chemical poses a terrorism threat at a time when Bush administration officials have issued warnings about "inevitable" attacks on U.S. soil.

Ever mindful of Mexican sensitivities to U.S. intervention, embassy officials say the United States has not played a major role in the search, although monitoring of the situation and "some collaboration" have been done. Officials in Puebla state say neither the FBI nor any other U.S. agency has contacted them.

"We have been collaborating with Mexico on their investigation and they're doing a great job," said Ingrid Larson, a U.S. Embassy spokeswoman.

Sodium cyanide is described as one of the most dangerous chemicals in the mining industry, which uses it to extract gold and silver from excavated rocks. If inhaled, ingested or absorbed through the skin, it attacks a person's nervous system and can cause suffocation.

Experts have told the Mexican media that the amount of missing sodium cyanide could kill 200,000 people if it was deposited into a public water source.

Much of Mexico's sodium cyanide is imported, although DuPont Corp. manufactures some in a plant near Puebla.

On May 10, the truck was on its way from Queretaro state to a mining company in neighboring Hidalgo state, a few miles north of Mexico City. The robbery occurred on a highway that was not part of the driver's planned route.

At first, authorities thought the thieves were interested in the truck, a 2002 model, and that they may not have known about its contents. But a week later, they found the truck abandoned near Zacatlan in Puebla state, with 83 of its 96 blue barrels of sodium cyanide gone.

One of the 13 barrels left behind had been opened. The company that owns the cargo, Degussa-Huls Mexico, offered a $2,700 reward for information leading to its recovery.

Among the theories circulating is that the chemical was stolen for resell to a mining company or hijacked by a mining company. Police suspect the barrels may be hidden in a mine shaft or at some remote ranch, although a search of three ranches near Zacatlan turned up nothing.

More than 100 agents and three helicopters have been deployed in the hunt. Most of central Mexico's municipalities, including Mexico City, have been put on alert.

Melgarejo did not rule out that the missing barrels might never be found.

"It could be," he said. "If they had a buyer in the industry, it's possible that no one will ever know who was the seller and who was the buyer."