While I'm sure more information will become available as an investigation is conducted, there's currently a fair amount of circumstantial evidence pointing to the Russian/separatist forces having shot down the plane. The plane was flying at just over 30,000 feet, which is out of range of the
that the separatists have most frequently been using, and requiring something like a [url=http://en.wikipedia.org/wiki/Buk_missile_system]BUK system. Earlier separatist forces
reported they had taken control of such a system, but a few hours after MH17 went down they pulled down their social media posts claiming such and started denying they had the capability to shoot down a plane at that altitude. Additionally, an An-26 (Ukrainian military cargo plane) was shot down three days ago while flying at a bit over 20,000 feet (generally outside the range of MANPADs), further supporting that separatists have the capability to shoot down planes at that altitude (or that Russia is willing to do it for them).
There was also
tweet released by [url=http://en.wikipedia.org/wiki/Igor_Girkin]Igor Girkin (a separatist leader) shortly after MH17 went down, claiming responsibility for just shooting down an An-26 in the same region that MH17 had just gone down (one current hypothesis is that separatist forces shot down MH17 thinking that it was another Ukrainian An-26). Translated:
Near Torez (city) an An-26 airplane was just shot down, it crashed somewhere beyond "Progress" mine site. We warned beforehand - do not fly over "our zone". Here's another video proof of the next "birdfall". A birdie crashed down beyond terricon (mine excavation site), missed living areas. Population not harmed. We also allegedly have info on another SU-25 shot down.
As a related note, Igor Girkin (aka Igor Strelkov) is former Russian GRU and FSB (reportedly active up until 2013).
Ultimately not conclusive, and it's always possible a false flag operation is in effect, but currently the actions of the separatists/Russians are not showing them in a good light. It also wouldn't be the first time that Russian military forces shot down civilian aircraft (see
and [url=https://en.wikipedia.org/wiki/Korean_Air_Lines_Flight_902]KAL-902, although to be fair such events have also occurred with
[url=http://en.wikipedia.org/wiki/Iran_Air_Flight_655]countries as well).