Board Thread:Questions and Answers/@comment-1362555-20140508130257/@comment-10556445-20140828112327

I think they're going with the IDW origin, where Hamato and his sons were told by Tang Shen to flee from Oroku Saki, who later found them in New York and killed them, and they were later brought back from the dead in the bodies of a rat and four turtles, where they escaped and found Mutagen spilled on them.