A very misleading assessment which conveniently leaves out evidence that doesn't fit.
// Ukraine also operates SA-11 systems, but we are confident no Ukrainian air defense systems were within range of the crash. //
After it became evident that the plane was a civilian airliner, separatists deleted social media posts... about possessing a Buk (SA-11) SAM system. //
The rebels captured a BUK launcher from the Ukrainians on the 29th June and boasted about possessing it on Twitter then. The way the 'assessment' is written makes the Rebel Tweets about about possessing a BUK were written after the delivery of a Russian BUK, but the boasting Tweets were actually written 3 weeks before (when they won one in combat).
The US 'assessment' neither admits the Ukrainians had lost a launcher to the Rebels or that it was likely to be in that area. Such an omission is on purpose, because if there were two launchers, we cannot be sure it was the Russian one which made the fatal error. Simpler to just ignore the existence of another launcher so that it is easier to condemn Russia.
http://www.liveleak.com/view?i=31f_1405616881