Medical researchers suspect that two North Korean defectors who resided near the Punggye-ri nuclear test site may have been exposed to radiation, though it is unclear whether nuclear tests are to blame, South Korea’s Ministry of Unification (MOU) announced Wednesday.

Korea Institute of Radiological and Medical Sciences (KIRAMS) identified the two defectors while conducting radiation exposure check-ups on 30 North Korean defectors at the MOU’s request.

The North Korea defectors volunteered to take the radiation checks, which were conducted between October 24 and December 6.

All the volunteers came from Kilju County, North Hamgyong Province, where the Punggye-ri nuclear test