In the world of criminal and constitutional law, custodial investigation and preliminary investigation are two different concepts that should not be confused.
Custodial Investigation
When you say custodial investigation or interrogation, it refers to the questioning initiated by a police officer AFTER a person has been taken into custody or deprived of his freedom of action. The key word here is deprivation of freedom. When a person is arrested, the arresting officer must inform him of his constitutional rights or more popularly known as the Miranda rights. He should be informed of the reason for arrest, the constitutional right to remain silent
where any statement made may be used against him and the right to communicate to a lawyer or any family member or relative. It is also important to take note that in this stage, the presence of a lawyer is required. Such right to counsel may be waived however, it should still be done in the presence of a counsel otherwise, the waiver has no effect at all.In case any of the constitutional rights mentioned are not properly observed and the accused made statements that may be used against him, it will be treated as INADMISSIBLE, meaning it has no effect and cannot be used against him in court. It's as if the accused did not say anything.
After such investigation, it shall be put into writing where the accused will signify that he attest to the truthfulness of the report. However, the contents must be explained especially when he does not know how to read or write.
Preliminary Investigation
On the other hand, preliminary investigation is a proceeding to determine whether there is a sufficient ground or well-founded belief that a crime has been committed and the accused is probably guilty and should be held for trial. At this stage, it is still NOT a trial since its purpose is to determine the existence of a probable cause. There is no need to present any evidence to prove or disprove guilt at this point since only a quasi-judicial officer is conducting this.
The right to preliminary investigation is a substantial right although the absence of such does not render the Information or complaint against the accused as defective. To afford the accused of its right to life and liberty, the trial court must order the prosecuting officer to conduct a preliminary investigation.
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Source: Basic Criminal Procedure by Antonio Bautista (2010)