How do I remove this freehub?

Hi, I trying to remove this freehub from an hold Trek bike. I have tried with a 9 mm Allen key but it goes through completely.
However with the 10 mm it reaches a point in which it stop (if inserted from non drive side) and grips something, but it doesn't really unscrew.
I suppose 11 mm would be too big given that 10 already can't go through completely.

Thanks!