Three main options:
1) Ukrainian quadcopter flew hundreds of miles.
2) Russians PO’ed about their crappy government and their stupid war
3) The Russian government did it themselves.
Personally, #3 makes the most sense to me. Caiming that Ukraine launched an assassination/terrorist attack against Dear Leader is good propaganda, and it gives the Kremlin permission – as if it really needs it – to launch further extermination campaigns against Ukraine. If evidence can be manufactured to show that the drones came from, say, Germany or Finland or the US, that gives that much more evidence that Mother Russia needs to wipe out the west.
There are, of course, other options:
4) The Chinese did it. Why? For the same reason Red China hired SPECTRE to develop reusable launch vehicle technology in the med-60’s in order to snatch both US and USSR capsules from orbit: to spark a war between the other two powers. China stays out of it, then moves into the chaos and rubble afterwards.