Actually the separatists have been showing off their buk missile system all week, and bragged about taking down their third plane today, but when they found out it was a commercial airliner, pulled back that statement.Separatists don't have the military hardware to pull this off. Only the Russian and Ukrainian Militaries do.
In situations like this, it is prudent to ask, "cui bono, who benefits?".
Russia has no reason to target civilian aircraft, and no benefits would come from such an attack. Where as an event like this benefits the Ukrainian Government greatly, they may now get the NATO/UN intervention they have wanted for months to finally quell the separatists in the East and bring Donetsk and Luhansk under their control.
IF that is true, and can be proven, Putin is in a world of hurt.
Maybe not. Rebels confiscated a Ukraine one in an attack a few weeks ago. It was a Russian BUC, but property of Ukraine. Wouldn't be hard to find experienced Russian operators. Perhaps some old veterans.