Board Thread:Manga/@comment-31209938-20170202034645/@comment-24407809-20170202202919

CommonSwift wrote: Probably the other kids from the warrior project, sold at the tener age of 5-6 so that their family could live as an honorary Marley, think of them as Kamikaze...or maybe some desperate soul who wanted their family to be taken care of in exchange of their sacrifice.

Marley only controls 7 Titan Shifters (including Zeke, Annie, Reiner, Bertolt, and Marcel), so there's no way all of the Titans were shifters. They were all mindless.

It has been hinted that Zeke is able to transform humans into Titans remotely, so my assumption is that he took a bunch of Eldians from Liberio (or any other Internment Zones) to Shiganshina and remotely transformed them when it was time for the battle to start.