SEOUL, South Korea — For two years, from a cramped office in central Seoul, activists and volunteers from five countries have been doing something never tried before: creating interactive maps of places where North Korea is thought to have executed and buried prisoners.

Over the years, defectors from the isolated country have testified to widespread human rights violations there, including what United Nations investigators have determined were extrajudicial executions of political prisoners and others. But there has never been a coordinated attempt to locate where those killings took place because the country remains off limits to outside rights investigators.

On Wednesday, activists affiliated with the Transitional Justice Working Group, a human rights group based in Seoul, announced their initial findings, identifying more than 300 sites where executions are thought to have occurred and 47 sites believed to have hosted cremations and burials, places where as many as 15 people may have been executed and their bodies dumped together or left “like trash.”

“It is not unreasonable to assume that mass grave sites in existence today will still be there years from now,” said Sarah A. Son, the group’s research director. “As we have seen in many other post-conflict settings around the world, people will want to know what happened to family members, and an accurate historical record will need to be created as part of the process of recovery, particularly on a scale such as that in North Korea.”