I don't think there has ever been a definitive answer, and I think that is the point. However, in my favourite screen version of that story - Jack Clayton's monochrome masterpiece The Innocents (1961), which is one of my all time favourite horror films - the screenplay (which Truman Capote polished) leans more towards a supernatural explanation (whilst still maintaining ambiguous wiggle room). I highly recommend it, if you haven't seen it.