The Buk, also called SA-11 or SA-17, is a large ground-to-air missile that can reach a maximum altitude between 11,000 and 25,000 metres depending on the version.
Guided by a radar station on the ground, it would be powerful enough to bring down a large aircraft with a single hit,
but not accurate enough to discern between a civilian airliner and a military transport aircraft, experts say.
This type of missile is known to be operated by both the Russian and Ukrainia armed forces, the International Business Times reported.
Larry Johnson, a former CIA official with counterterrorism experience, told Associated Press that if the rebels have a Buk system it is
entirely possible that they could have mistaken the civilian airliner for a military transport aircraft before they fired a missile.
"The Buk uses a radar acquisition system for targeting," he said. "These aren't highly trained FAA air traffic controllers. You're tracking something on radar,
you see a dot, you get confused. I don't think it was deliberate. I think it was mistaken identity."
MH17: What is a Buk missile launcher? | Stuff.co.nz
The Associated Press reported seeing the weapon - which resembles a launcher mounted on top of a tank - in the eastern Ukrainian town of Snizhne earlier on Thursday.
Justin Bronk, with the Royal United Services Institute, told AP he believes that if a Buk SA-11 system was used, he is "almost certain" it was supplied by Russia.
"My personal hunch is that given the military setbacks that the separatists have suffered of late, and the Ukrainian military's increasingly confident use of airpower, Russian authorities decided to send a few SA-11 systems across into the Donetsk area," he said.
"However, I also highly suspect that the separatists did not intend to shoot down an airliner, but probably thought they were targeting a Ukrainian transport at high altitude, he said.
I think this is exactly what happened, the Ukraine had air superiority, the Russians decided to even the odds by giving the seperatists BUKs, which is perhaps more conservative than using Russian fighter planes to counter the Ukraine air forces.