Ten Russian troops were captured in August in eastern Ukraine amid fighting between pro-Moscow separatists and Ukrainian troops after weeks of Moscow denying involvement in the Ukrainian conflict.
Activists have collected data about dozens of Russian servicemen who were killed in recent months.
Authorities insisted that the captured soldiers got lost while patrolling the border while the deaths were accidental and happened in Russia.
The Soldiers' Mothers of St Petersburg said today that its colleague in southern Russia who was investigating the deaths and disappearances was detained today.