Deadly mobile SAMs, or surface-to-air missiles.
Russia has thousands of them. So does Ukraine.
And, according to a witness who spotted at least one armoured vehicle bristling with four missiles in a rebel-held town in eastern Ukraine earlier this week, Russian separatists fighting to secede have at least a few, too.
Radar-guided and mounted on armoured military vehicles, the needle-nosed Soviet-era missile known as Buk can reach into the stratosphere and kill as far as 60 kilometres away.
Just such a surface-to-air missile – with a warhead and range far bigger than the shoulder-fired missiles used to shoot down low-flying helicopters and military transports in eastern Ukraine in recent weeks – almost certainly destroyed the Boeing 777 over eastern Ukraine.
Malaysia Airlines Flight 17 was far too high to be brought down by gunfire, and shoulder-fired missiles can rarely reach above 3,000 metres.
No one yet knows exactly what brought the airliner down. Although it might have been a bomb or some extraordinary mishap like a meteor strike, the probable cause of the crash of Flight MH17 from Amsterdam to Kuala Lumpur – unlike most air disasters – seemed evident almost immediately.
All sides, even as they denied responsibility and accused the other, agreed that the airliner – with 298 people on board – was hit by a surface-to-air missile. And in the increasingly dangerous skies over eastern Ukraine, the Boeing 777 would be only the latest, albeit the worst, in a deadly spate missile attacks on aircraft.
Something suddenly shattered the plane as it cruised more than 10 kilometres up, ripping it apart so the twin-engined, wide-body, intercontinental jet broke up at altitude, scattering parts and victims along a grim trail of debris many kilometres long. The tail broke away, leaving the bulk of the fuselage to crash in a fiery explosion that sent an oily black plume of smoke into the afternoon sky.
No call came from the cockpit of the stricken aircraft, suggesting that whatever doomed it was catastrophic, leaving no time for even a Mayday.
Investigators poring through the wreckage will likely find bits and pieces of a surface-to-air missile, probably from Buk systems dating back to the 1970s and known to NATO depending on their variants as Gadfly, Gantry and Grizzly.
Ukraine denied that its military, currently waging a tough campaign with tanks and heavy artillery against rebel strongholds, fired on any aircraft. Ukrainian President Petro Poroshenko blamed separatist rebels, calling them terrorists. "The armed forces of Ukraine did not take action against any airborne targets."
Andrei Purgin, one of the rebel leaders, said he was certain Ukrainian troops shot down the Malaysian jet. Mr. Purgin said he did not know whether separatist forces had been given or captured surface-to-air missiles capable of reaching high altitudes but, he added, even if they did, they lack the expertise to operate them.
Yet suspicion was directed at the rebels for another reason.
The pro-Moscow separatists have shot down several Ukrainian helicopters and at least one Ukrainian Antonov-26 military transport in recent weeks. The Donetsk People's Republic, one of the separatist militias, claimed to have shot down another Antonov-26 on Thursday about the same time Boeing 777 crashed. It remains unclear whether Ukraine's air force actually lost another aircraft or whether the incident involved the Malaysian jet.
If missile debris can be found, it should be possible to identify the manufacturing batch and trace when it was made, sold and deployed, and whether it was among munitions captured by or given to the rebels or if it remained in Russian or Ukrainian inventories.
Ukraine and Russia have shot down civilian airliners before.
In 1983, high-ranking Soviet military commanders ordered fighter pilots to intercept and then shoot down an off-course Korean Airlines Boeing 747 they believed was a U.S. spy plane. All 269 on board were killed.
In 2001, an errant Ukrainian surface-to-air missile shot down a chartered Siberia Airlines Tupolev-154 carrying Russian Jews to Israel off the Crimean Peninsula during military exercises. All 78 on board died.
The U.S. military has also shot down a civilian airliner. In 1988, the warship USS Vincennes mistook an Iranian Airbus for a warplane and shot it down with a surface-to-air missile over the Persian Gulf, killing all 290 on board.
Contact was lost with the Malaysian Flight 17 about 40 kilometres west of the Ukraine-Russia border, well within range of some surface-to-air missile systems even if the launch was from inside Russia, although nothing suggests that was the case.
Locating the launch site may be possible. NATO and U.S. E-3 Sentry aircraft have been patrolling the skies west of Ukraine. Their radar may have picked up the brief – 2,000-kilometres an hour – flight of the missileas it streaked skyward toward its target.
U.S. spy satellites capable of photographing single military vehicles may also have spotted something. Whoever fired may now be trying to destroy the launcher.
A picture said to have been taken in the past week of a Buk-class military launcher next to an apartment building in eastern Ukraine has surfaced.
Identifying either the missile used from debris or the launcher will be powerful evidence in the search for who shot down the airliner.