Who was the only head of the Soviet Union's secret police to die of natural causes (in 1934)?

Don't give us the year. It's not as much fun or challenging to research. AFT should be about interesting research questions.

That would be Menzhinsky. See below:

https://en.wikipedia.org/wiki/Vyacheslav_Menzhinsky

Vyacheslav Menzhinsky was the only Soviet secret police chief to die of natural causes back in 1934, during Stalin's reign. He led the OGPU, the main security and intelligence agency at the time. Unlike many of his successors, who were later executed or purged, Menzhinsky escaped that brutal fate due to poor health and staying largely out of the spotlight. His death stood out in a time when holding power in Soviet security was often a death sentence.

https://en.m.wikipedia.org/wiki/Genrikh_Yagoda