Here you go ATA:from Osprey Aircraft of the Aces, LaGG & Lavochkin Aces of World War 2.
"There were two more kiils scored by the leading ace which the Soviets never credited, and which Kozhedub was more than happy to forget. In April 1945 he saw an American B-17 being attacked by a pair of German fighters. Coming to the bomber's rescue, he fired a burst between the enemy machines and their intended prey, causing the Germans to break off their attack and dive away. Before Kozhedub could pursue them, he spotted an approaching formation of unfamiliar fighters. The leader of the group opened fire on Kozhedub at long range, and the Russian ace in turn pulled up sharply behind the last machine in the formation and shot it down in flames - the fighter fell among Soviet troops on the ground. Pulling up in a half loop, Kozhedub then fired another burst into the leader of the formation, who was also shot down. It was at this point that he saw the blue and white stars on the wings and fuselage of his 'enemy'. Kozhedub returned to base extremely upset, certain that his actions would result in a major row with the allies. Fortunately, one of the American pilots managed to bail out, and when he was asked who shot him down, he replied 'a Focke-Wulf with a red nose'. Col Churikov gave Kozhedub the gun camera film confirming his two Mustang kills, along with the admonition, 'Keep this to yourself - show no one'."
So you were right ATA, the Americans did fire first.
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