Usually, police like to say any investigation is ongoing, especially when criminal matters are pending.
However, usually, as far as the investigation goes, they work it until they determine there's probable cause to arrest a certain suspect. They've arrested two who were obviously the shooters, who may have actually confessed themselves to the shooting (everyone is assuming KJ "snitched" but there's no proof of that).
Once they have their suspects, they get warrants for their arrest. And they've done that here. Two suspects arrested.
If Joseph was being fingered for being an accomplice, then what else would they be waiting on to arrest him? He was there. Everyone knows it. He even admitted it. Yet, no arrest.
That's because they decided not to arrest him, for whatever reason. Mostly likely because he wasn't involved in the shooting.