They have the name of every black saturn astra 4-door hatchback owner in the country on June 9th, 2017. There are more than a few, but all things considered not that many. They will start in Illinois and then gradually move out to neighboring states and beyond if need be. All of them will be questioned at some point. Their friends, family, co-workers, and associates can be questioned if needed. It is almost guaranteed they will find the <modsnip> white male who was in the area and driving that vehicle, if they haven't already.
My gut tells me that since there isn't a desperate media push for more information, and police haven't released anything new in 10 days, they probably already have a suspect. They may just be building their case or waiting to find Yingying. I feel like if the investigation were stuck, there would be a more frantic public effort on behalf of LE to get more tips and information. That doesn't seem to be the case.