Malaysia plane MH17: Game-changer in Ukraine crisis?
For a passenger airliner to suffer a catastrophe in mid-flight is extremely unusual.
To down a modern passenger jet typically either something has to explode on board, or, as looks increasingly likely in this case, it has to be shot down, either from the air or from the ground.
The Ukrainian authorities seem in no doubt as to what has happened.
An adviser to the Ukrainian interior minister, Anton Herashchenko, alleges that the plane was flying at an altitude of some 10,000m (33,000 ft).
He says that it was hit by a missile fired by a Buk launcher.
The Buk, also known by its Nato codename of Sa-11 Gadfly, is a Russian-made, medium-range surface-to-air missile system designed to defend against high-performance aircraft and cruise missiles.
Buk surface-to-air missile system
Also known as SA-11 Gadfly (or newer SA-17 Grizzly)
Russian-made, mobile, medium range system
Weapons: Four surface-to-air missiles
Missile speed (max): Mach 3
Target altitude (max): 22,000 metres (72,000ft)
Source: Global Security
The Sa-11 is a more capable successor to the older Sa-6 system.
Typically, four missiles are carried on board a tracked vehicle which also houses the guidance and tracking radar.
A separate tracked vehicle carries the Snow Drift target acquisition radar.
Everything will now depend upon obtaining accurate and verifiable data on the aircraft's last moments and on what brought it down.
If it does turn out that the Boeing 777 was shot down by the separatists - with weaponry supplied by Moscow - then it could significantly alter the terms of the whole debate surrounding the Ukraine crisis.
Over the past few days there has been growing concern among Western governments that Russia was stepping up its military support for the separatists in eastern Ukraine.
Nato spokesmen insist that more and more heavy military equipment has moved from Russian stockpiles to the separatists across the border.
In response, the United States has strengthened its economic sanctions against Moscow - it is threatening even stronger action - though the European Union has so far failed to follow Washington's lead.
But if Russia in any way had a hand in this tragedy then the pressure - especially on the Europeans - for much tougher sanctions will only grow.
It will also prompt a renewed push for a diplomatic solution to the crisis.
Moscow clearly does not want the separatists to be swept aside.
But equally Russian President Vladimir Putin seems to have no clear game-plan as to what the end-state should be.