When investigators form theories about anyone's guilt, the result can be tunnel vision and can lead to miscarriage of justice. Investigators might ignore other possible leads or interpretations of evidence.
For example, once an investigator becomes convinced that a particular person committed the crime, they might subconsciously disregard or downplay any evidence that contradicts their theory. Over time, this narrowed focus can result in biased decisions, preventing the truth from surfacing.
Investigators don't look for evidence to support their theories. They have to look for evidence without a rush to judgement, while remaining objective and without bias or preconceived theories of who might be guilty.
I agree, any investigation must start with an open mind and a wide search for any clues, through intense seaching, not just for the victim, but for any physical clues such as tracks, phone data, dropped toys or clothing, distant CCTV capturing a vehicle, publicizing for tips, a nearby dingo den....
On the other hand, everything criminology has learned about missing children is that the most likely perp is the family member or close friend of family.
Similar to how, if anyone's wife disappears, the husband will be the prime suspect. Just because he hires a lawyer and refuses to speak to police, no way are LE going conclude 'ok, the guy hired a lawyer so I have to presume he's innocent, and try to find some other perp'.
IMO, that is not strictly speaking tunnel vision it's a process of ruling out the most obvious suspects, just based on the nature and circumstances of the crime.
- the person was there on the scene
-the person has no alibi.
I'm sure if other evidence comes up, they'll follow it - but, in the absense of any other evidence, who else would be on their suspect list? They won't start just investigating random people for no reason.
JMO