We have become used over the past few months to a constant stream of intelligence from US satellite and airborne surveillance, channelled through the Ukrainian authority. It has effectively monitored the size, composition and formation of Russian units on the border, has spotted the movement of just three tanks across the border from Russia to Ukraine and has provided Ukrainian units with detailed intelligence on the armour and other fixed defences of the rebels. It is quite inconceivable that the US did not know of the presence of a SAM 11 (BUK) air defence system deep in rebel territory - and particularly so if its target-acquisition radar had been active.
They question they must answer is who did they tell. No airline in the world would continue to route flights over an area if it had been warned explicitly of the risk of a live SAM 11 system in unstable hands. So just who did the US tell?