This is exactly how Russia operates inside Europe.

A leftist group cannot be sponsored by Russia?

How much sponsorship does it take to commit arson?

Who cares? The amount is irrelevant.

Why would they be? Russia doesn't tend to fund leftist groups. If they're actually a leftist group, it's far more likely to be about Palestine and of their own accord, or because of direct government oppression against them.

Last time I knew of an attack like this in Berlin, it was a direct retaliation after the government attacked their house with military tanks.

I would be especially concerned if the government attacked with non-military tanks.

The government dropping a water tank on your house certainly would be concerning.