"A civilian airliner traveling from Amsterdam to Kuala Lumpur has just been shot down by a Buk anti-aircraft system," Ukrainian Interior Ministry official Anton Gerashchenko told Interfax-Ukraine news in the wake of today's tragedy, in which a Malaysian Airlines passenger jet carrying 295 people was destroyed, and all its passengers killed.
That account of what brought Malaysia Airlines Flight 17 down has not yet been confirmed, and the Russian separatists Gerashchenko was implicating have denied responsibility. The AP reported seeing a Buk in the rebel's possession, in the same region, earlier this week.
The above video is among the only I could locate that shows a live fire test of the weaponry in question.
The war machine was initially developed by the Russians in the 70s, and first deployed on the battlefield in 1979. Wikipedia explains it thusly:
The Buk missile system is a family of self-propelled, medium-range surface-to-air missile systems developed by the Soviet Union and its successor state, the Russian Federation, and designed to engage cruise missiles, smart bombs, fixed- and rotary-wing aircraft, and unmanned aerial vehicles...
A standard Buk battalion consists of a command vehicle, target acquisition radar (TAR) vehicle, six transporter erector launcher and radar (TELAR) vehicles and three transporter erector launcher (TEL) vehicles. A Buk missile battery consists of two TELAR and one TEL vehicle. The battery requires no more than 5 minutes to set up before it is ready for engagement and can be ready for transit again in 5 minutes. The reaction time of the battery from target tracking to missile launch is around 22 seconds.
The Buk's radar can track aircraft flying up to 72,000 feet and guide up to three missiles towards it at once. Needless to say, operating the machine is highly technical work, as each TELAR requires a team of four trained personnel. Without those trained personnel, it would theoretically be easy for a poorly-trained soldiers to mistake a commercial airliner for a military transport vehicle.
That, right now, is a leading speculation from a number of observers—that this extremely deadly weaponry was being wielded by badly prepared separatist fighters, who may have been armed by Russia, and who mistakenly fired a deadly payload towards an airline full of innocent passengers.