Trustworthy information is very scarce
but I don't think that plane was shot down by flak. It appears to have been at cruising altitude, and it suffered a catastrophic failure. My guess is it was out of range of AA guns or shoulder-fired rockets.
My guess is it was taken down by high altitude air defense missile, fire from an intercepting aircraft, or an onboard explosion.
My guess is that the "official" investigation will conclude it was just an unfortunate accident or mechanical failure.