Human rights advocates on Tuesday urged South Korea to offer radiation exposure tests to hundreds of North Korean escapees who had lived near the country’s nuclear testing ground.

Tests conducted by the South Korean government on 40 people in 2017 and 2018 found at least nine of them had abnormalities that could indicate high radiation exposure, but Seoul’s Unification Ministry said a conclusive link to North Korea’s nuclear activity couldn’t be established and other factors were possible, such as age, smoking habits or other types of chemical exposure.

The South Korean radiation tests were subsequently discontinued.

The Seoul-based Transitional Justice Working Group cited the findings and its own analysis of geographic and census data to say that North Korea’s six nuclear detonations could have spread radioactive materials by water within 40 kilometers (24.8 miles) of the Punggye-ri nuclear facility. It said more than a million people live in the area dependent on groundwater and wells since piped water is scarce beyond the capital, Pyongyang and a few other cities.

North Korea has rejected safety concerns surrounding its nuclear tests, saying the testing environment each time was fully controlled and that it detected no radioactive leaks. It allowed foreign journalists to film the detonation of some tunnels at the site in 2018 but has never allowed international nuclear inspectors to visit the Punggye-ri testing ground.