Another one of my random musings (which is likely wrong). Maybe they DO have him in custody and are in the process of interrogating him and don’t want the others in the cell to know. Seems odd that this 19 year old kid could escape a massive police chase and search, when hardened criminals can’t manage to evade a few squad cars in a getaway.
My guess is the cops are scared shitless. This kid and his brother are wired to suicide vests and were lobbing grenades at the cops. They're not military (no matter how much they try to dress like Infantry or SpecOps guys), so they're not used to this kind of resistance. I'm betting that they're giving him a lot of leeway while still trying to contain him.