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

Question

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 1

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 ... ?'

Answer 2

sawa's answer explains the の, but I feel のか as used in the question hasn't been fully explained.

It doesn't correspond well in this situation to "is it the case that...?", because that is a question that demands an answer. This particular use of のか doesn't. I doubt it would be said with a questioning tone.

"So you have that/those kind(s) of doubt(s)/suspicion(s)/question(s) (about it), huh."

I believe this is sense 二[7] of か in Daijisen:

７　引用した句の意味やある事実を確かめ、自分自身に言い聞かせる意を表す。「急がば回れ―」「そろそろ寝るとする―」

and sense 四[9] of か in Daijirin:

⑨ 独り合点の気持ちを表す。詠嘆・回想の気持ちが強い。
「『春はあけぼの』か、いい文句だな」
「そうか、失敗だったのか」

The word "ponder", which Ignacio Vazquez-Abrams used in a comment to the original question, seems to fit quite well as a description of what this か is doing.