BUK misssile system
In court testimony about MH17 tragedy Russian official reveals Ukraine deployed 17 BUK missile launchers to eastern Ukraine.

The Russian authorities have for the first time revealed in court testimony the full extent of Ukraine's deployment of BUK missile systems to eastern Ukraine when MH17 was shot down.

The Russians have previously produced satellite photos of some of these launchers, the authenticity of which has sometimes been challenged (for example by the Bellingcat site in Britain) but which is now generally acknowledged. These satellite photos have however never received wide publicity in the West. On one occasion I found when I mentioned their existence to a British MP that he knew nothing of them.

The Russians' disclosure of the extent of Ukrainian BUK missile deployments to the war zone in eastern Ukraine on the day that MH17 was shot down was made by Ilya Rogachyov, a Russian Foreign Ministry official, in evidence to the International Court of Justice in The Hague, where Ukraine is bringing a case against Russia. This is what Rogachyov is reported to have said:
It is noteworthy that in the summer of 2014 Ukraine's 156th air defense regiment armed with Buk-M1 systems was in the area of the conflict. The regiment's command center and the first unit were in the area of Avdeyevka, near Donetsk, the second unit was near Mariupol, and the third, near Lugansk. All in all the regiment had 17 Buk-M1 systems at its disposal. All were identical to the one the JIT identified as the weapon that downed the air liner.
The earliest report I know of to suggest that MH17 might have been shot down by a BUK missile launched by Ukraine's 156th air defense regiment was one which appeared on the Voice of Russia website on 25th July 2014 sourced to RIA Novosti. It reads as follows (bold italics added):
The Malaysian Airlines MH17 that crashed in southeast Ukraine could have been brought down by an accidental launch of a missile by Ukrainian forces during an anti-aircraft exercise, Russia's RIA Novosti agency reported, citing an unnamed source in the Ukrainian defense ministry.

"On July 17 the commanding officer of 156th Anti-Aircraft Regiment was instructed to conduct a training exercise of ground troops stationed near Donetsk, which involved deploying the troops, and carrying out a routine tracking and destroying of targets with the Buk-M1 missile," the source said.

The source added that the actual launch of the missiles was not intended.

Two Sukhoi Su-25 combat aircraft participated in the exercise. It is likely that at some point, the routes of the Malaysia Airlines Boeing 777 and a Su-25 jet overlapped.

Despite flying at different levels, they became a single dot on the radar of the missile system. Of the two, the system automatically chose a larger target.

The reasons for the actual missile launch - if indeed it did occur - are under investigation. Practical exercises with the Buk missiles have been banned since 2001, when a Russian Tu-154 passenger airplane en route from Novosibirsk to Tel Aviv was shot down by the Ukrainian military.
I should say that I have little doubt that this RIA Novosti article, which claims to be based on information obtained from within Ukraine's Defense Ministry, originates from Russian intelligence.

It might be based - as it claims - on an actual source within Ukraine's Defense Ministry - presumably an informer or a spy - in which case it merits serious attention, which to my knowledge it has never received. However even if the article does originate from a source within Ukraine's Defense Ministry, that does not mean the claims it makes are true.

The 156th air defense regiment is the Ukrainian air defense unit that the person who calls himself "Andrew" has also identified as having deployed BUK missile launchers to the war zone of eastern Ukraine at the time when MH17 was shot down. Since "Andrew" draws extensively on Ukrainian sources, the claim Ukraine's 156th air defense regiment with its BUK missile launchers deployed to the east Ukrainian war zone in the summer of 2014 appears to have been proved beyond reasonable doubt.

The fact Ukraine deployed 17 BUK missile launchers to the war zone of eastern Ukraine in the summer of 2014 does not mean that it must have been one of these launchers which shot down MH17. However the Western public's impression of the tragedy would surely have been very different if Ukraine's deployment of these systems was widely known about. As it is the Western public has barely been told about them to this day.