The Ukrainian government has announced, and there is video evidence and journalist reports confirming, that two Ukrainian SU-25s were brought down in the vicinity of the crash site of MH-17. The fighters came down around five miles from the Russian border near the town of Dmytrivka (see The New York Times map on its Ukraine Crisis in Maps page). According to the spokesman for Ukraine’s Defense and Security Council, Andrey Lysenko, the planes were flying at an altitude of 5,200 meters (a little over 17,000 feet).

If, and I stress if, it is true that the planes were at 5,200 meters when they were hit, then it is likely, as the Ukrainians are suggesting, that they were brought down either by surface-to-air missiles (SAMs) fired from the Russian side of the border or by air-to-air missiles fired from a Russian attack fighter or fighters.

As discussed in my post on MH-17, the two kinds of missiles known to have been in the possession of the separatists before last week are various types of MANPADS and the Strela-10 truck mobile SAM. The Igla-type MANPADs, with its relatively small explosive charge would have a hard time bringing down the armored SU-25 under the best of circumstances, but more importantly, both the Igla (in any of its forms) or the Strela-10 have a maximum intercept altitude of no more than 3,500 meters. Accordingly, some other SAM would have had to have brought down the two Su-25s.

It is possible that the separatists still have Buk missiles, launchers, and command vehicles in their possession, and that the separatists brought down the Su-25s using Buks fired from territory under their control. At this point, it seems highly likely that it was a Buk missile, which has a maximum range of up to 72,000 feet, fired by the separatists that brought down MH-17 last Thursday. However, there is now a great deal of convincing video, photographic, and eyewitness evidence that the Buk missiles and launchers were driven out of Ukraine and across the border into Russia in the early hours of Friday morning, shortly after the crash of MH-17. This makes sense. If it was indeed a Buk missile fired by the separatists from the Ukrainian side of the border that brought down MH-17, the separatists would have every incentive to get them out of the conflict zone and onto Russian territory immediately. So it seems quite unlikely that there are still Buks in their possession in eastern Ukraine.

If that is the case, then the only alternative explanation for today’s incidents is that the Su-25s were brought down by SAMs fired from the Russian side of the border or by Russian fighters. If the former, there are two possible ways it could have happened. The Buks driven across the border on Friday could have remained under the control of the separatists and deployed near the border, presumably camouflaged to keep them from being detected by U.S. satellites. If so, it is clear that this could only happen with cooperation from the Russian border guards and the Russian military – as I have argued before, Russia is not a country where people drive tanks, let alone Buk missiles, around on their own.

However, this scenario seems implausible because it is not clear what the purpose of having separatists control and fire SAM missiles across the border would be. What seems more likely is that the Russian military is using its regular, more sophisticated, air defense systems – for example, S-300s (SA-10) or S-400s (SA-21), with their integrated radar systems – to carry out the Kremlin’s instructions to keep the supply lines open to Luhansk and Donetsk, and it is using SAM missile strikes to keep the Ukrainian air force from attacking those supply lines. The S-300 and S-400 also have a very large intercept range – around 195 kilometers and 400 kilometers respectively, according to Wikipedia – so they could be very effective in denying Ukraine control of the airspace over the conflict zone if Russia were to choose to use them openly and to target planes well inside the border. It is also possible that Russian fighter jets have been given instructions to engage Ukrainian fighters near the border.