Outwardly, nothing was amiss when a 40-foot-long container was unloaded last February at the Port of Los Angeles after an 11-day voyage aboard the cargo ship Hanjin Mokpo from South Korea.

The container was stacked on the docks, and a day or so later was hauled about 20 miles to a Compton warehouse, where clerks unpacked the cargo, including a battered plywood crate. Soon the crate was loaded on a truck for a three-week, cross-country trip.

It wasn't until the crate reached its final destination at Amersham Corp., northwest of Boston, that an alarm sounded, warning that the cargo contained potentially dangerous levels of radiation.

The incident touched off a far-reaching federal investigation into how the shipping crate slipped undetected through the harbor, crossed the country by truck and exposed dozens of unsuspecting people to a pellet of Iridium-192, a radioactive isotope typically used to X-ray the strength of industrial welds. It also has rekindled debate over the safety of shipping radioactive materials on the nation's highways.

The crate contained 14 stainless steel carrying cases designed to transport radioactive materials used for industrial purposes. The shippers believed that the cases were empties being returned to Amersham, which manufactures and distributes radiographic materials.

What workers who handled the crate on the docks, at the warehouse and in the truck didn't know was that one of the supposedly empty cases contained a radioactive pellet.

They were also unaware that somewhere en route to Massachusetts the pellet had become dislodged from the side of the carrying case designed to shield the public from radiation.

A 150-page report released in May by the U.S. Nuclear Regulatory Commission concluded that the shipment of the pellet was inadvertent and that no one was exposed to doses of radiation large enough to significantly increase their risk of getting cancer. But it cautioned that the pellet "had the potential to cause high radiation exposure to members of the general public."

After tracking the crate's movements, investigators placed the blame on the South Korean shippers for allegedly failing to heed U.S. government safety rules by not labeling the crate and its contents as radioactive. There were no warning signs on the truck that took the cargo cross-country--through parts of Los Angeles, Orange and San Bernardino counties as well as the cities of Albuquerque, Oklahoma City, St. Louis, Baltimore and Philadelphia.