Here are the words of a Portuguese Law expert (my emphasis):
Detectives invoke arguido status on someone as a preliminary to an arrest being made or charges brought, a Portuguese law expert, Lita Gale, told Guardian Unlimited today.
"If you are an arguido they
have to have suspicion that a crime has been committed by that person," she said.
(My note- in order to have a 'suspicion, one needs evidence against the person- no evidence, no suspicion)She added that when someone's status moves from that of a witness to an arguido, the police interview technique changes.
"Depending on the answers they may be moving towards a prosecution. If you cannot provide an alibi or the right answers they may move to prosecute. This is a serious moment. The suspect may be close to being charged," Ms Gale said.
Technically, police should treat someone as an arguido as soon as they have sufficient evidence. However, in some circumstances they may hold back to give somebody "a false sense of security", she added.
In practical terms, a person has to be declared an arguido before he or she can be arrested. Police may also ask a local court to place restrictions on a person's movement and oblige them to not leave the country.
Once a case file is completed, the police pass it to the public ministry, the equivalent of the crown prosecution service, which decides whether an "acusacao" or indictment is brought.
(My Note: The case file is a summsry of all the evidence applicable to the case)
http://www.guardian.co.uk/uk/2007/sep/07/ukcrime.madeleinemccann2