The commercial jet Malaysia Airlines flight MH17 crashed over the southeastern border of Ukraine yesterday, killing all 298 people aboard. The US government confirmed the tragic news late in the afternoon, hours after the plane crashed, saying a surface-to-air missile brought it down. By that time, there were already several accusations about who was responsible for the act, and several vehement denials. Members of the Ukrainian government accused pro-Russian separatist rebels in the region. The most influential rebel group in the area disavowed the claim. The Russian government blamed the Ukrainian government for allowing flights in the area. As of Friday afternoon, the true perpetrators are still unknown.

"Evidence indicates that the plane was shot down by a surface-to-air missile."

President Obama noted that US intelligence indicated the missile had been fired from an area in Ukraine controlled by pro-Russian separatists and that these rebels had received support from the Russian government. "Evidence indicates that the plane was shot down by a surface-to-air missile that was launched from an area that is controlled by Russian-backed separatists inside of Ukraine," Obama said. "Moreover, we know that these separatists have received a steady flow of support from Russia. This includes arms and training. It includes heavy weapons."

Looking at the facts so far, it’s possible to narrow down a list of those groups that may have access to the weaponry needed to take down a Boeing 777 jet flying at an altitude of 33,000 feet, as Malaysia Airlines flight MH17 was. "Both the Russian military and the Ukrainian military have significant quantities of the SA-6 / SA-17 series of missiles," says Christopher Harmer, a retired US Navy aviator who now works as senior naval analyst at the Institute for the Study of War, a nonpartisan think-tank in Washington, DC.

The SA-6/SA-17 refers to a variety of surface-to-air missile manufactured in Russia. Also known as the Buk or Grizzly, it is a vehicle-mounted missile system that was first developed in the 1970s under the name SA-6 to defend the Soviet army from NATO warplanes, according to The Daily Beast. Since that time, the Russian government has sold and given the missile system to other governments of the Soviet Union, including Ukraine, and many have ended up on the black market. "We can’t prove that Russian separatists in Ukraine were given these weapons by the Russian military, but they may have simply taken them from Ukrainian bases when the Ukrainian military withdrew," Harmer explains. "They could have picked them up in Crimea and moved them to Eastern Ukraine. They could have bought them on the international black market….regardless of how the Russian separatists got the SA-6 / SA-17, it seems pretty clear that they have them now."

International media reports have pointed to one separatist group in particular as having both the weaponry and the will to shoot a plane out of the sky: The Donestk People’s Republic, a well-armed, pro-Russian separatist faction that came to power in the region of Ukraine near the plane crash in May. More specifically, reports have singled out one military commander of the Donestk People’s Republic, Igor Girkin (aka "Strelkov," Russian for shooter), as inadvertently claiming responsibility for the incident.

"The plane could easily have been mistaken for a military aircraft."

An account on the popular Russian social network VKontakte claiming to be Girkin's posted a message on Thursday showing video of the plane crash and saying "we just downed a plane, an AN-26." The message, which was quickly deleted, has led many to speculate that Girkin’s fighters accidentally shot down Malaysia Airlines MH17 after they mistook it for a Ukrainian military cargo plane. "The plane could easily have been mistaken for a military aircraft," says Harmer, citing the 1988 incident of Iran Air flight 655, in which a US Navy warship shot down an Iranian commercial flight, believing it to be a military aircraft. "If the US Navy can mistake a commercial passenger flight for a military attack aircraft, then it is certainly reasonable to expect that Russian separatists would make the same mistake," he adds.

Lending support to this theory is the fact that another AN-26 cargo plane was shot down three days earlier, allegedly by some of the same pro-Russian rebels. Yet as Vox points out, it’s unclear if the VKontakte account that posted the message on Thursday actually belongs to Girkin or any of his direct associates.

But there’s more circumstantial evidence that points to pro-Russian rebels in Ukraine as being responsible for the attack: the Ukrainian government late Thursday released audio of several intercepted phone calls allegedly between pro-Russian rebels and Russian military officials. On one call, a rebel military commander affiliated with the Donestk People’s Republic, Igor Bezler, claims to have shot down a plane. On another call, two unidentified men discuss the debris from the crash, saying they have found "civilians" and "civilian items," including "medical stuff" and an Indonesian student’s ID. They swear as if they are upset.

"It is also possible that Ukraine or Russia made some form of similar error."

However, leaders of the breakaway Donestk People’s Republic have denied shooting down the plane or possessing the necessary weaponry to do so. Some rebels have even suggested that the Ukrainian government is responsible. Complicating matters, Malaysia Airlines released a statement noting that a Ukrainian air traffic controller instructed MH17 to lower its altitude to 33,000 feet, just above prohibited airspace.

Other experts are not willing to rule out the idea that Russia could have taken down the plane by accident. As Anthony Coresman, a US defense expert at the Center for Strategic and International Studies, another DC-based think-tank writes: "It is also possible that Ukraine or Russia made some form of similar error." He goes on to point out the issues with these possibilities, though. "The basic problem is the vector," Coresman continues. "The Ukraine would have seen that aircraft was flying into Russia and already over rebel-held territory when the missile was fired. Russia would have seen a single plane flying well above military operational altitudes on a vector that would not match any Ukrainian military operation. But human error does happen, particularly when both sides may be on the edge of overreacting and have virtually no real operational experience."

For now, the White House continues to focus on pro-Russian separatist groups in Ukraine as the likeliest suspects, with possible involvement from the Russian government. Although many questions remain about the tragedy, that a surface-to-air missile was used seems beyond dispute at this point. As for how the US government knew that such weapons were used, Harmer says: "Each missile has a unique thermal signature. A satellite in geosynchronous orbit over Ukraine can be passively watching; if it sees a missile plume with the correct thermal signature, we can say, ‘That plume at location X is a SA-17 missile plume.’"