  We classify binary completely regular codes of length $m$ with minimum
distance $\delta$ for $(m,\delta)=(12,6)$ and $(11,5)$. We prove that such
codes are unique up to equivalence, and in particular, are equivalent to
certain Hadamard codes. We prove that the automorphism groups of these Hadamard
codes, modulo the kernel of a particular action, are isomorphic to certain
Mathieu groups, from which we prove that completely regular codes with these
parameters are necessarily completely transitive.
