On May 9, 2019, at 1228 eastern daylight time, a Cessna C550, N744AT, experienced a total loss of engine power inflight and landed in Savannah, Georgia, without further incident. The two airline transport pilots, two medical crew, and three passengers onboard were not injured. The flight was operated as a Title 14 Code of Federal Regulations Part 135 air medical flight.The airplane was based at Punta Gorda Airport (PGD), Punta Gorda, Florida. According to fueling records from PGD, on the morning of the incident, the airplane was fueled with 480 gallons of Jet A fuel with a fuel system icing inhibitor (FSII) additive mixed at the time of fueling. The flight crew completed the 47-nautical-mile flight from PGD to Naples Municipal Airport (APF), Naples, Florida, without anomaly. According to the pilots, about 1 hour 20 minutes into the subsequent flight, from APF to Niagara Falls International Airport (IAG), Niagara Falls, New York, while cruising at 35,000 ft mean sea level (msl), the pilot-in-command (PIC) adjusted the left engine N1 speed to about 103%. Moments after adjusting power, the N1 speed decreased. This occurred "a few times" before the left engine began to "spool down very slowly." After unsuccessfully attempting to restore engine power, the crew requested a lower altitude from air traffic control and began a descent with the left engine at idle power. The PIC then noticed that the left engine displayed no oil pressure, and he subsequently shut it down. Several minutes passed as the airplane descended with the right engine at 65% fan speed, and while the crew prepared to perform a single-engine approach to Savannah/Hilton Head International Airport (SAV), Savannah, Georgia, about 8,000 ft msl, the right engine became unresponsive and then began "spooling down." The captain declared an emergency and the crew performed a straight-in approach to runway 19, where they landed the airplane without incident; the airplane was subsequently towed to the ramp. The second-in-command noted that the left fuel filter bypass light did not illuminate, but the right fuel filter bypass light did illuminate. A lineman who worked for the fixed based operator (FBO) at PGD stated that, the evening before the incident, he noticed that the FSII was low on a fuel truck and he intended to refill it. He went to a shed where the FSII was located and noted that the unmarked FSII container was partially filled, and another bottle next to it that was also partially filled. He combined the two containers and then refilled the fuel truck FSII reservoir. Several days after the incident, the lineman realized that he had inadvertently combined the contents of a 5-gallon FSII container and a 2.5-gallon diesel exhaust fluid (DEF) container instead of two partially empty containers of FSII. He could not recall if the DEF container was marked. Fuel samples, fuel system filters, and fuel screens from the incident airplane were obtained and sent for laboratory testing. Analysis of the fuel indicated the presence of urea; DEF is a solution comprised of about 33% urea and 67% water. Although urea is soluble in both water and FSII, when added to hydrocarbon jet fuels, it forms a solid precipitate (known as a "clathrate") that is known to interfere with fuel filtering and fuel system operation. Review of the lineman's training records indicated that he completed National Air Transportation Association (NATA) Fuel Service Supervisor training in July 2018, Professional Line Service training in September 2018, and Line Fuel Service training in September 2018. His training records did not specifically include the DEF Contamination Prevention training released by the NATA in December 2017; however, the lineman did state that he had received DEF training and cross-contamination training in June 2018. The FBO policies in place to mitigate the hazard of DEF contamination before the incident included isolating FSII to the self-serve shed, isolating DEF off-airfield in the vehicle maintenance shop, conducting mandatory training on DEF contamination, and reviewing cross contamination with staff. After the incident, the FBO made multiple changes to their policies, including retraining line personnel, limiting DEF servicing to vehicle maintenance personnel, removing all unmarked containers from storage areas, and creating a monthly inspection program to review all storage facilities for properly stored and labeled inventory. On July 24, 2019, the NTSB issued Safety Alert SA-079 warning providers of jet fuel to take measures to prevent contamination of jet fuel by diesel exhaust fluid.