You are probably correct, it looks like they just ran out of time.
I just reread this from the transcript:
GR: Do you have any evidence that Caylee is alive? Other than what Casey told you?
CN: Well, at this point, we don't have any information provided to us that leads us to believe otherwise. We believe that she's still out there. That she can be found. If anyone provides us any information. But again, the information we're receiving, the evidence, the follow up that our detectives are conducting with the assistance of the FBI lab as long with the Federal Department of Law Enforcement and their crime analysis folks, receiving all of those tips of information, we have not been able to establish her well being.
If I am reading this correctly, Captain Nieves is actually saying that taking into account all the evidence they have so far, they have not been able to establish Caylee's well being.
Any thoughts?