I think the suspicious part is that no one was at the front desk. This is a secured building. No one can get through the front desk unless they have permission, but if there is no one manning the front desk than anyone could walk in
A person could have broke into the guys apartment, killed him, and then lit the apartment on fire uniformly. Stuff was falling through the vent….could have been an attempt to spread the fire.
The fire could have been set to gain access to the residence. Firemen or SS one or both could have been there to plant something bug / evidence or something.
Cohen calling his friend suggests possible prior knowledge.
OR the witness was lying to set Cohen up thus giving FBI plausible reason to raid his office.
OR Cohen saw fire on the news and just called his friend like anyone would