definitions - のか - Particle "lamenting reflections on the preceding statement". What?

I've come across this sentence in my manga.

そ　そうか　そういう疑いを持っているのか

It's actually fairly easy, but the particle at the end has confused me. I thought it was the two particles の and か, but then I saw this in a dictionary:

のか - Particle - endorsing and questioning the preceding statement (sentence ending particle); lamenting reflections on the preceding statement (sentence ending particle)

That seems to fit, because it's at the end of the sentence, and he's commenting on what another character said to themselves. But beyond that, I have little idea of what this means. Is it actually a question particle similar to か? When would you use this instead?

Answer

This の is called a formal noun, and has a very general meaning that can be translated to English as 'the case', 'the fact', or 'the situation', and it heads an appositive clause. It is the same の used in the のだ consruction. It typically implies that the appositive clause is an explanation to the previous context. It has pretty much similar implications that the English the case that ... implies.

... の だ
'It is the case that ... .'

... の で ...

'Things being the case that ... , ... .'

... の か
'Is it the case that ... ?'