GQ’s Charlie Burton interviews Stephen Turnbull about ninjas:
“If the ninja have any basis in fact, the following three criteria must be satisfied,” says Turnbull. “One, that a unique corpus of military techniques involving secrecy existed in Japan during the Sengoku Period [c. 1487 – 1603]. Two, that the exercise of these techniques was confined to certain skilled individuals rather than being spread more widely within Japanese society. And three, these skilled practitioners were identified in particular with [the areas] Iga and Kōka, from where they sold their services to others.”
The title of the piece is a bit misleading. It isn’t examining whether ninjas exist currently, but whether they ever existed.