It's reported that Malaysia Airlines flight 17 broke apart in the air by high-energy objects from outside. The flight came down in eastern Ukraine in an area controlled by pro-Russian seperatists and the US and Ukraine have accsued the seperatist of downing the plane with a missile. However, The Dutch Safety Board believes there was a warhead exploding near by the passenger jet but it is unknown who fired it.
I believe it was the Pro-Russian separatists who downed the plane because they are basically well armed guerrilla forces that don't have to answer to anybody. The have no legitimate ties to Moscow, and so they could risk shooting down a passenger plane. If Ukranian forces shot down a passenger plane, they could lose their NATO support.
It had to have been the Pro-Russian Separatists. According to the article, the separatists took immediate control of the crash site "combing through the wreckage and hindering access 'to investigators.'" What are they looking for? If they claimed not to have shot the plane down, then why would they refuse access to investigators? It makes no sense.
Like Sami said, I believe Pro-Russian separatists are behind the downing of flight 17. They have no allegances to any one country, therefore, they do as they pleased. It is a horrible situation. No country wants to loose NATO support, because that could mean the end of them. I don't believe any one country or countries purposefully shot down flight 17. The downing of flight 17 in my opinion was the result of radical separatists who need to be contained. Much similar to ISIS in the sense that they are provoking a lot of unneeded casualties.