If Japan is having a civil war, and up to that point there has been consistent shelling and troops against its own people for years and then a chemical weapon went off and lots of civilian dies, then yes, the signs would point to the government in charge.
I mean people might as well say it was the US who fired the chemical weapon in
Syria to create and excuse to invade.
Uhh have you not read the news, there are "accusations" of a merc group planting the device, at the behest of the US government.
Of course such things are unsubstantiated...kinda like any evidence thus far.