Alright, so this is partly speculation, but looking at things as they've happened and taking a step back, here's what I see as the big picture:

Russia begins ramping up attacks against ISIS, so IS bombs a Russian commuter plane.

Russia is un-fazed, continues bombings, targeting specifically the fleets of oil trucks that IS has been using to smuggle crude to middle men who sell it to other markets.

All of a sudden Turkish fighters rapidly down a Russian warplane that they claim violated their airspace after "10 warnings within 5 minutes without response."

Russian-Turkish geopolitical relationship begins to tank, and so I start researching the Turkish leadership.

According to this article, Bilal Erdogan (son of the Turkish President) was educated at Harvard before returning to Turkey to start his Marine transport business. Said business has been traveling to Syria for commerce since April 2014, at least.

Then in May of this year, an article was published in Turkish media showing alleged photographic and video evidence of trucks with ties to the Turkish intelligence apparatus smuggling weapons into Syria. Said weapons were at first stated to be headed to Kurdish rebels to be used against Assad, but that was later vehemently denied. Those reporters were recently arrested.

All in all, there is very real credence to the Russian claims of Turkey's implicit aid to the Islamic State. The question now, in my view, becomes: How much have NATO, the CIA and the US State Department been aware of and/or a part of all this?

And I think we may actually be able to find ties between them.