Make no mistake, [imho] they have at least one POI. If nothing else, as evidenced by the warrant of the workplace. I assert they don't bother to search one's office (nor get judge to issue a warrant for same) without being
very interested in you, or something about you.
The key is they (presumably intentionally) haven't
named a POI (as that requires them to divulge other pieces of information to the defense earlier than LE would prefer to). It may be frustrating to the public, and may appear as if there's little progress, but it doesn't mean that's reality.
It's semantics, and strategic: If LE thinks that publicly naming someone a POI will help them get to an arrest sooner (perhaps force someone to "slip up", get "nervous", or confess), then they'll name a POI/suspect.
If LE doesn't think naming a POI/suspect will help them get to an arrest sooner, then there's no incentive to name one. [ At least not in the short term, and even in the long term, it just becomes a PR problem ("LE's been working on this case for XX and they still don't have any leads/suspects!"), but the PR is still secondary to their desire to achieve justice (and rightly so IMO) ]
In this specific case, it seems fairly likely that a suspect/POI will be named/made-known at the same time they're being placed under arrest. Even then, they may not do it explicitly. [ Though, I'll also assert it can be inferred when being read your Miranda rights, that LE has a developed a profound interest in you
]
LE has POI(s), they just haven't (and likely) won't name them (does not imply a lack of progress [let's hope]
).