Could it perhaps have something to do with the fact that surprisingly and oddly, the parents were less than helpful to the inquiry this having been borne out by Kate McCanns less than enthusiastic comments about the Portuguese police in her book?
Is it normal for the parents of a 'disappeared under suspicious circumstances' child to thwart the police investigation and refuse to answer ALL their questions?
A one word response will suffice.
At that stage of the investigation the police were intent on pinning the disappearance of the child on the child's mother. Much the same tactic had already 'solved' the case of another missing child.
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Several similarities between the cases—both girls vanished without trace within 7 miles (11 km) and less than three years of each other, in both cases officers failed to secure the crime scene, both mothers mounted campaigns to find their daughters and both women were accused of involvement—prompted Joana's family to appeal in 2008 for police to investigate whether there was a link between the disappearances.[5]
https://en.wikipedia.org/wiki/Murder_of_Joana_CiprianoThere is absolutely no doubt in my mind that Kate McCann took precisely the correct lawful course of action in implementing her right to silence ... and I rather suspect you know that she did particularly as she had just been subjected to a round of eleven hours of police interrogation.