User blog comment:God of nothing/Titan shifters-Invincible?/@comment-5972544-20131001192939

The can be killed, in their human form. For example, Mikasa could have killed Reiner and Bertholdt by cutting off wither of their heads, moreover, Eren could have killed Annie by eating her.

Of course, this hasn't been comfirmed but its highly likely to be true.