Reports of a crashed Malaysian airliner are still coming in — follow the Washington Post's liveblog here — but there are already suggestions that MH17, which lost contact with officials near the Russia-Ukraine border, was shot down by surface-to-air missile.

According to flight data, the aircraft was flying at altitude just before the crash, at roughly 33,000 feet.

To take down a plane at these heights would require an advanced air defense system; the Ukrainian Interior Ministry says the separatists who fired the shot were armed with a Russian-made Buk missile launcher.

The Buk, also known as the SA-11, is a medium-range mobile missile platform that can engage targets flying at altitudes well above MH17's last reported figures — by some estimates, as much as 72,000 feet. Newer versions of the Buk are known as the SA-17; here's a video of one in action.

Ukrainian National Security and Defense Council spokesman Andrei Lysenko told Interfax Thursday that the Ukrainian army is not equipped with the SA-11, which raises questions about where the separatists may have gotten one (if that's in fact what they used). For their part, rebel leaders deny having any Buk missile launchers. But other militia members told Russian state media last month that they had "taken control" of a number of SA-11s.

Another possibility? Man Portable Air Defense Systems, or MANPADS. These are lighter and can be carried by a single person. In fact, separatists have been using shoulder-fired missiles for weeks to shoot down other aircraft, according to aviation specialist David Cenciotti. But even the most advanced MANPADS have a limited ceiling compared to bigger missile platforms. The SA-18, which the separatists have reportedly used before, has a maximum altitude of 11,000 feet.

On top of the plane crash itself, it appears we now have a puzzle concerning the provenance of the weapons that caused it.

Update: U.S. officials have confirmed that MH17 was downed by a surface-to-air missile, according to The Post's own Ernesto Londoño.

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