MEXICO CITY >> Six people being tested for possible radiation exposure in a hospital in central Mexico are suspects in the theft of highly radioactive cobalt-60, a government official said today.

The official said the six were arrested Thursday and taken to the general hospital in Pachuca for observation and testing for radiation exposure. Once they are cleared, they will be turned over to federal authorities in connection with the case of a cargo truck stolen Monday at gunpoint that was carrying the extremely dangerous material.

The official spoke on condition of anonymity because he was not authorized to speak to the press. He did not specify how the six were allegedly involved in the theft.

Hidalgo state Health Minister Pedro Luis Noble said Friday that the six suffered from skin irritations and dizziness, but that none are in grave condition and may be released soon. Only one was vomiting, a sign of radiation poisoning.

But based on the tests, “none are showing immediate signs of radiation poisoning,” Noble told Foro TV.

The cobalt-60 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.

The atomic energy agency said the cobalt has an activity of 3,000 curies, 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.”

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 government official would not give details or location of Thursday’s arrest nor names or ages of the suspects.

The six were arrested by Hidalgo state police, said state attorney general’s spokesman Fernando Hidalgo.

The driver of the truck, who had stopped to rest at a gas station early Monday when the theft occurred, said two armed men made him get out, tied his hands and feet and left him in a vacant lot.

Hidalgo said he didn’t know how or if the others were involved.

The truck was found abandoned Wednesday about 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 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.

Authorities maintained a 500-yard cordon around the site where the cobalt-60 still remains in the state of Mexico and continued to work Friday to extract it safely, said Juan Eibenschutz, director general of Mexico’s National Commission of Nuclear Safety and Safeguards.

“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.”

Federal police blocked access Friday to hospital where the six were held.