On 9 May 2019, a Cessna 550 level at FL 350 experienced an unexplained left engine rundown to idle and the crew began descent and a diversion to Savannah. When the right engine also began to run down passing 8000 feet, an emergency was declared and the already-planned straight-in approach was successfully accomplished without any engine thrust. The ongoing Investigation has already established that the likely cause was fuel contamination resulting from the inadvertent mixing of a required fuel additive with an unapproved substance known to form deposits which impede fuel flow when they accumulate on critical fuel system components.