Hello! In my opinion, the investigators are obviously investigating something so a better word would be to "figure" out as they will be looking at all the evidence.

I'd say that it is possible to say 'sort out' in this context, but it isn't the best phrasal verb to use here. 'Sort out' generally means organise or deal with. It can be used to mean 'resolve' in the sense of find a solution to a problem, but it isn't usually used to mean 'discover'. You could, as Kim suggests, say 'find out'. 'Find out', meaning 'discover', is OK. It's still not the best phrasal verb for this context, however. The clue to this question is the word 'investigators': this tells us that they collected evidence and then analysed this evidence until they finally understood why the accident happened. To describe this process, we use the phrasal verb 'work out' (more common in British English) or 'figure out', as Kim suggests (more common in American English). If this is a textbook example from a British publisher, my money is on 'work out'. If it's American, 'figure out' is more likely.

We would say find out not sort out. Find out means to discover.

Yup! Sounds good! =) You may add "the" in-front as well: "The investigators are still trying to sort out why the accident happened." Hope I helped! =D