How we really really knew the Russians did it: the intel services didn't just have smoking guns, they had real-time tracking of the bullets in flight! Fascinating reporting from the Dutch, whose intel guys handed it all to us, wrapped up with a pretty pink bow:
https://www.volkskrant.nl/media/dutc...m_content=free
(Lots of Dutch on the page, but the story is in fairly-decently translated English....)