Notice some key things here.

The U.S. declared that Ukraine was not involved, but that "extremists" were the culprits.

The article even mentions Chechens.

So, how likely is it that what we see here is maybe Russian/Chechen pro-Ukraine types infiltrating into Ukraine to conduct terror operations.

That way Ukraine can claim deniability.

And it fits with Nuland's parting gift, of some "special surprises for Russia".

https://www.bbc.com/news/live/world-68642036?src_origin=BBCS_BBC