First, the plane was shot well inside the Ukrainian territory. The "Buk" system cannot operate at such ranges if it were deployed on the Russian territory.
Second, the Ukrainian army does have Buk systems. And they are notorious for erroneously shooting down a plane coming from Israel to Russia some 10-15 years ago. However, I can hardly believe they did it, unless anybody went totally mad or planned a provocation to blame Russia.
There were unconfirmed rumors that the Ukrainian rebels in the south-eastern part of the country have recently got 1-2 Buk complexes after their successful offensive towards some (Ukrainian) army locations (home bases). In a similar way they got some IFVs and artillery systems earlier because the army is very weak. If the latter is true and the system is operative and in use, then most likely the rebels thought it was a transport aircraft of the Ukrainian Air Force. They shot down one or two of them very recently.
Anyway, there should be E-3 Sentry and also the Russian A-50s monitoring the conflict area from far away, so the records should exist. Also, the flight recorders should not be damaged.