Putin's top tank commander Maslov dies suddenly after a meeting was canceled.
Maslov was criticized by Putin and Kremlin for not producing enough tanks for the Ukrainian war.
Russia's Federal Security Service investigates Maslov's sudden death.
Maslov is the latest in a string of mysterious deaths of Putin's allies.
Former general Alexander Buzakov also died suddenly on December 24.
Sausage tycoon Pavel Antov died under mysterious circumstances in India.
Russian man Vladimir Budanov died from a heart attack in India during a vacation.
Both Antov and Budanov criticized Putin and the Ukrainian war before their deaths.