[She shrugs at that.]

Not everyone here is human, so he might have always been a skeleton. I just know that something happened, and he started killing people.

[It's possible that he just... snapped? Although, to be honest, Robin sort of doubts that theory.]

I think they just welded it shut. I don't know, though - this all happened before I showed up. The other captains seemed pretty confident that he couldn't get out, though.