Or they just let it happen then blame Ukraine because they will have more reasons to take more action against Ukraine. Some news sources say the US had reported to Russia about this incident, but Russia seems not to care. By the way, it's just in the media so we don't know the truth.
Well. Since we are talking about Russia and those who are involved seem to have been captured by the Kremlin, it is likely we will never know for sure what the truth is.
If some of those terrorist got captured by other state which happened to be more transparent and did not have reasons to blame a neighbor country, then there would be more chances for us to know legitimately what is going on. Russia is like that, there is a strong hermetism on the official information. It is not at the same level of North Korea, where people cannot literally move out their motherland, but there is indeed control of the information.
We can only know the little facts we can trust in order to assemble the puzzle as much as possible and reach our own conclusions.