A small container of radioactive material that was stolen last week in southeastern Mexico was found Wednesday abandoned under a pedestrian bridge, resolving the fourth such theft since 2013.

The April 13 theft of the toolbox-sized container carrying the Iridium-192 capsule in Cardenas, Tabasco state, had triggered an alert in five southern and eastern states and the activation of federal forces.

Officials believe that, like in the previous three cases, the thieves were unaware that they had stolen a radioactive item and were more interested in stealing other goods. The material was recovered every time.

The capsule was recovered under a bridge on a road linking Cardenas with Tabasco's capital Villahermosa, after someone notified state police by phone, authorities said. Some 100 security forces were deployed for the recovery.

"There are no signs that the container was opened," Christian Romero, deputy director for radiological emergencies at the national nuclear commission, told AFP.

Officials have stressed that the capsule posed no threat as long as it is kept under seal. But they advised the public to stay well away from the material if it is found.

National civil protection coordinator Luis Felipe Puente said the Iridium-192 was "under guard" after the nation's nuclear safety commission confirmed that it was recovered.

Iridium-192 is used for industrial radiography to check welding seams.

It can cause burns, radiation sickness and permanent injury if a person comes in contact with it, and can be fatal if exposure lasts hours or days.

The theft was reported by the company Garantia Radiografica e Ingenieria. The robbers took the container along with other objects inside the radiology company truck, which had been parked in a residential area of Cardenas.

The thieves left that vehicle alone and fled in another stolen truck.

- Previous radioactive alerts -

Nuclear commission officials say companies are not required to have a security detail accompany material like Iridium-192. Security forces do escort more lethal material.

The biggest scare for Mexico came in December 2013 when thieves took a truck containing a cancer-treating medical device with highly radioactive cobalt-60 near Mexico City.

Authorities arrested and hospitalized five suspects in that case after recovering the potentially lethal material, which the thieves intended to sell as scrap metal. They all survived.

That theft prompted the International Atomic Energy Agency to issue an alert for "extremely dangerous" material while US officials kept tabs on the situation.

In February authorities recovered three stolen trucks in central Mexico transporting radioactive material for industrial use.

A similar incident took place in July 2014, also without causing harm, a day after a truck was stolen containing a highly toxic and flammable material.