The gangs in your area must be different than in our area. Here..they don't contain themselves to a specific part of the city. Their crimes range from gang on gang to home invasions of anyone and everything in between. Armed robberies, rapes, murders, and car theft/jackings are to them what going to work is for everyone else. It supports their drug habits, their gang activities, and creates a citywide terror. It is more than difficult to get anyone to testify against them. You know without any doubt you are risking the lives of your family. They have proven it repeatedly.
I hope they find out who murdered this sweet girl. I thought they had a good idea at first, but evidently that didn't pan out. Have they mentioned sexual assault? It is the one thing that could go towards motive. If the other person in the car were trying to just rob/kill her...they could have done that without driving to that area. I think it might be possible she drove the car into the lake on purpose trying to get away from him. It is possible the struggle was ongoing at that point and he finished the job before he bolted.
I don't see the reason why she would be outside of the car unless she was still alive when the car hit the water. She either got out on her own or he pulled her out after she was already gone. Makes more sense she got out on her own...because it would have been in his best interest for her to go into the lake inside the car.