Complex sentence: There have been investigations by both the Soviet, and Belarusian governments, which have been inconclusive as to the perpetrators who are believed to be either the Soviet NKVD, or the invading German forces.
Split into simpler sentences: There have been investigations by both the Soviet, and Belarusian governments, which have been inconclusive as to the perpetrators. Believed either to be the Soviet NKVD, or the invading German forces.