grammar - Use of の with かもしれない

北海道の海岸に向かう途中で、川に入ってしまったのかもしれません。
They might have ended up entering the river en route to the Hokkaido coast.

What is の (in bold) doing in this sentence? It was my understanding that かもしれない came straight after the predicate.

My guess is that this is the so-called 'explanatory の' rather than a nominaliser. Please can you confirm or correct my thoughts?

Answer

Yes, this is a typical example of explanatory-の. Looking at the source, we can see this sentence is trying to explain why the アザラシ was found in the mountain.

• 川に入ってしまいました。
  The animal ended up entering the river.


• 川に入ってしまったのです。
  It is that the animal ended up entering the river.


• 川に入ってしまったかもしれません。
  The animal may have ended up entering the river.


• 川に入ってしまったのかもしれません。
  It may be that the animal ended up entering the river.