PACHUCA, Mexico (AP) — Federal police blocked access Friday to a central Mexico hospital where six people were reported to have been admitted with radiation exposure.

An official familiar with the case confirmed Mexican media reports that the six have been admitted to the general hospital in the city of Pachuca and may have been exposed to a stolen source of highly radioactive cobalt-60.

The official, who spoke Friday on condition of anonymity because she was not authorized to be quoted by the media, said only one person so far was dizzy and vomiting, symptoms of severe radiation poisoning. She said two people were admitted on Thursday and four more Friday. She did not provide any other details.

It was not clear if the people under observation were also the thieves, who early Monday stole a cargo truck at gunpoint that was carrying the radioactive material in Hidalgo state, where Pachuca is located.

The federal attorney general's office, which is handling the case, could not be reached for comment on whether suspects were in custody.

The theft triggered alerts in six Mexican states and Mexico City, as well as international notifications to the U.S. and the International Atomic Energy Agency in Vienna. It raised concerns that the material could have been stolen to make a dirty bomb, a conventional explosive that disseminates radioactive material. But Mexican officials said that the thieves seemed to have targeted the cargo truck with moveable platform and crane, and likely didn't know about the dangerous cargo.

The atomic energy agency said the cobalt has an activity of 3,000 curries, or Category 1, meaning "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."

"What I was told yesterday is that there might be two people with severe radiation syndrome, but I do not have confirmation," said Juan Eibenschutz, director general of Mexico's National Commission of Nuclear Safety and Safeguards.

Pedro Luis Noble, health minister for Hidalgo state, told Reforma newspaper that the people believed to be exposed pose no risk to other patients and that they are in an isolated area.

The truck was found abandoned Wednesday about 40 kilometers (24 miles) from where it was stolen, and the container for the radioactive material was found opened. The cobalt-60 pellets were left about a kilometer (half mile) from the truck in an empty rural field, where authorities said they were a risk only to anyone who had handled them and not to anyone in Hueypoxtla, the closest town of about 4,000 people. There was no evacuation.

The material was from obsolete radiation therapy equipment at a hospital in the northern city of Tijuana and was being transported to nuclear waste facility in the state of Mexico, which borders Mexico City.

Eibenschutz said authorities continued to work on Friday at the site in Mexico state where the material was found to extract it safely.

"It's quite an operation and it is in the process of being planned," he said. "It's highly radioactive, so you cannot just go over and pick it up. It's going to take a while to pick it up."

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Associated Press writer Adriana Gomez Licon reported from Mexico City.