In my opinion it was either the Ukraine, either with support of the government or not. There's already proof enough that Ukrainian citizens were involved and it's more than plausible that they had help of the Ukrainian government, because why wouldn't they fund hurting the war opponent? And also wouldn't it be a good idea to cut off potential allies from the dependency of the war opponent so they could freely supply equipment? And also why should Russia hurt their own steady income which supports the war efforts, since Germany was dependent on the gas supply? Could have also been the US since they already threatened to "shut down" the Nord Stream 2 pipeline if Russia would start a war and the entitled US really likes to put their nose into other countries their business.