Mark,
My two cents worth. I personally feel that there is always a presumption of innocence until all the facts are in. A person can be a "person of interest" without being a suspect. A homeowner, a parent, a juvenile, ect ect. Anyone with an interest in the case can be a person of interest. This is not a preconceived notion of guilt. Asking a person for an alibi is allowing the investigator to narrow the scope of his/her investigation. It's no different in the private world. If a piece of equiptment is being targeted as an ignition source and test reveal that the equiptment is not capable of producing the tempetures needed to start an ignition , could this be a scientific alibi? A flip side of this is that confession does not stand alone. That is why a throughly maticulous O&C needs to be preformed to either suppor of reject verbal information received. This is true on both the public and private sectors.
Nick