So it wasn't the magicians... it was the POLICE!
Fuck da police!
If I was a mage, I'd be telling everyone that it was the goverment too. However, magic-catching bureau running black sites not only on mages but on citizenry as well aren't making such plausiable deniability easy. But my theory works perfectly if higher-ups of the government and nobility are mages themselves. And they don't want competition. That's why they made a spechial governmental institution and gave it such a huge authority, that they can act more bluntly than gestapo did in their prime.
I wonder how they justify to the common people that they can be just shot at any point and these guys so far don't seem to even need to as much as write a raport. I need more global worldbuilding.