How did Igor Sergun die?

The Kremlin announced the death of Gen. Igor Dmitrievich Sergun, the director of the Main Intelligence Directorate (GRU) of the General Staff of the Armed Forces of the Russian Federation, on Jan. 4 with a short statement. It was reported that the cause of the sudden death of Sergun, one of Moscow's main men, on Jan. 3, was a heart attack. No further details were provided.