Mexican authorities Thursday recovered dangerous radioactive material from a cancer-treating medical device that was on a stolen truck and abandoned in a field, the interior ministry said.

It was in a capsule of two centimeters in diameter and authorities are now trying to isolate it safely before taking it to its original destination at a waste storage facility, the ministry said in a statement.

A family that handled the device is being monitored for health risks.

Soldiers had set up a 500-meter (yard) safety perimeter around the hazardous material after it was found 70 kilometers (43 miles) north of Mexico City on Wednesday, two days after the truck transporting it was stolen.

The radioactive cobalt-60 source, which is considered "extremely dangerous" by the United Nations' nuclear watchdog, was originally inside a device that was in a steel-reinforced box in the truck.

The material was found in the town of Hueypoxtla about one kilometer (0.6 miles) from the truck, which the driver said was stolen by two gunmen at a service station on Monday.

The theft raised concerns about health risks while experts warned that the quantity of cobalt-60 -- 60 grams -- was enough to build a crude "dirty bomb," though it was possible the thieves were only after the truck.

The United States said its national security team had monitored the situation "very closely" and that President Barack Obama was briefed throughout the day on Wednesday as the search was on for the missing material.

"We also took appropriate precautionary steps along our shared border with Mexico," said White House spokesman Jay Carney.

The National Commission for Nuclear Safety and Safeguards (CNSNS) said a family found the open medical device and brought it inside their home.

"We will have to keep this family under medical watch for the sole reason of being near a certain distance from the source," CNSNS operations director Mardonio Jimenez told Milenio television, without specifying how many members were in the family.

Authorities have warned that whoever removed the radioactive material by hand was probably contaminated and could soon die. Authorities were still looking for the thieves.

They said it is not clear if they are the ones who opened the box.

But Jimenez sought to reassure residents in the 40,000-population town of Hueypoxtla.

"The source is far from the population," he said. "There is a security operation to keep them from getting near it."

The official blamed the transport company for the incident, saying it had acted with "negligence" by not having a security escort with the truck. The device was driven from a hospital in the northwestern city of Tijuana.

The Vienna-based International Atomic Energy Agency also said the Mexican public "is safe and will remain safe."

The IAEA said it had been informed by the CNSNS that the cobalt-60 was found to have been removed from its shielding "but there is no indication that it has been damaged or broken up and no sign of contamination to the area."

The UN agency said that if not securely protected, the 60 grams of material "would be likely to cause permanent injury to a person who handled it or who was otherwise in contact with it for more than a few minutes."

"It would probably be fatal to be close to this amount of unshielded radioactive material for a period in the range of a few minutes to an hour," it said.

The IAEA added, however, that people exposed to the radioactive substance "do not represent a contamination risk to others."

The incident was a reminder of the dangers posed by the huge amounts of nuclear material in hospitals and industry around the world if they are not handled properly and with sufficient security.

In particular, there are fears that extremists could steal the material and put it in a so-called dirty bomb -- an explosive device spreading radioactivity over a wide area and sparking mass panic.