Most definitely, LE has the evidence to convict Casey on first degree murder! The grand jury only heard from 6 witnesses. We know that George and Yuri testified, but there were only 4 others.
When the State Attorney, Lawson Lamar, addressed the judge, having the judge sign the sealed indictment, he said a few words. I made notes, and this is one thing he said..........."the grand jury has seen things they shouldn't have to see. They've learned law and unfortunately, a lot about forensic evidence."
My impression after hearing the above sentences is that the jury saw a lot more forensic evidence than we're aware of. We know of a stain, a hair strand, and dirt in the trunk of Casey's car. We know that Casey checked out cloroform and websites for missing children online. But would any of these thing qualify for "things they shouldn't have to see."