South Korean scientists and doctors who have been examining North Korean defectors have stumbled upon yet another horrifying discovery: At least four of the defectors have shown signs of radiation exposure, the South Korean government said on Wednesday – although researchers could not confirm if the radiation was related to Pyongyang’s nuclear weapons program.

Earlier today, we noted that one of the defectors had also tested positive for Anthrax antibodies, suggesting that North Korean leader Kim Jong Un has continued his chemical weapons program despite signing an international chemical weapons treaty. Of course, the North Korean government has denied that chemical weapons are being used.

All four men are former residents of Kilju county, an area in North Korea that includes the nuclear test site Punggye-ri. According to Reuters, they were likely exposed to radiation between May 2009 and January 2013. All of the men defected to the South before the most recent test, according to a researcher at the Korea Atomic Energy Research Institute.

The researcher cautioned that people can be exposed to radiation in many ways, and that none of the defectors who had lived in Punggye-ri itself showed specific symptoms.

Fears that North Korea could unintentionally trigger a nuclear disaster via its nuclear tests have escalated since the North’s Sept. 3 nuclear test, prompting China to increase monitoring of radioactive activity along its border with its restive neighbor. Seismic activity detected in the aftermath of the test suggests the test site is suffering from “Tired Mountain Syndrome” – a condition detected at former Soviet nuclear testing sites. The destabilizing impact of the tests was made evident when a tunnel at Punggye-ri collapsed, killing 200 North Korean workers.

Still, the North has pushed ahead with building new tunnels at the test site, suggesting that – instead of abandoning Punggye-ri altogether, as their Chinese peers have advised, they intend to move the tests to a different part of the mountain.

Ultimately, scientists worry that the mountain could implode, releasing a plume of toxic radioactive dust into the atmosphere that could wreak untold havoc on the health of people across the region.