Board Thread:Manga/@comment-5774380-20161007195718/@comment-29098965-20161009001437

And just one more thing which bothers me. We do not now the details yet, but

Somehow Grisha had the titan serum which he injected to Eren.

Rod Reis had the titan serum which he wanted to inject to Crista ( sorry, Historia), but took it himself.

I don't know if there is another way to turn a human into mindless titan, but seems like Mare government has the serum as well. (Also considering that Zeke had it to turn the residents of a village into titans.)

I would theorize that maybe Grisha as a doctor was made to create the serum to turn his team Grisha to dumbdumbs. Then, he would be able to make the serum again (maybe) when living inside the walls. But what about Rod Reiss? Remember, when he was about to inject Crista, he said smth like, I have picked up the strongest titan for her. You can pick up only if you have smth to pick up from. When you have plenty to choose from.