Their last radio message was a request for vectors around a thunderstorm, which suggests to me that they had clear line of sight to the storm itself. For them to, in doing so, steer into the upper layers of a sandstorm would have been extraordinary bad luck. It was night-time so they wouldn't have seen it until they were inside it.
I'm anticipating the investigation will focus on the weater forecast issued to the pilots and how they arrived at their go/no-go decision.