It has been confirmed in the Russian media that the Syrians shot it down using an old S-200 SAM. The bodies and aircraft fragments and debris are being collected by the Russian ships.
The Israeli aircraft played a simple trick approaching the shore nearly along the line-of-sight between the S-200 and Il-20 and warning the Russians about their intended attack just 1 min in advance through a "hot line" (as Russian media say). It was reported that it was too late to remove a slow Il-20 from the dangerous area.
The Russians are angry about Israel. Anyway, the Syrians are to be blamed in the first instance, IMHO.