Short answer. Provincial or City Prosecutors and their assistants, judges of the Municipal Trial Courts and Municipal Circuit Trial Courts, National and Regional State Prosecutors, and other officers authorized by law may all conduct preliminary investigations of crimes within their territorial jurisdiction.
What the law says
The following may conduct preliminary investigations: (a) Provincial or City Prosecutors and their assistants; (b) Judges of the Municipal Trial Courts and Municipal Circuit Trial Courts; (c) National and Regional State Prosecutors; and (d) Other officers as may be authorized by law. Their authority to conduct preliminary investigations shall include all crimes cognizable by the proper court in their respective territorial jurisdictions.
Rule 112, Section 2 — Officers authorized to conduct preliminary investigations. Read the full provision →
Four categories of officers are authorized
The rule lists exactly who may conduct a preliminary investigation: Provincial or City Prosecutors and their assistants, Judges of the Municipal Trial Courts and Municipal Circuit Trial Courts, National and Regional State Prosecutors, and other officers as may be authorized by law. This is a defined, closed list, not an open invitation for any government official to simply take on the role.
Their authority is bounded by territorial jurisdiction
The authority granted is not unlimited: it shall include all crimes cognizable by the proper court in their respective territorial jurisdictions. A prosecutor or judge conducting a preliminary investigation is acting within the geographic and jurisdictional limits tied to their office, not investigating any crime anywhere in the country regardless of where it happened.
This territorial limit ties the officer's investigative authority directly to the specific courts whose criminal cases they would be helping to prepare for eventual filing.
The list leaves room for other legally authorized officers
The final category — other officers as may be authorized by law — acknowledges that additional bodies or officials can be given this authority by separate legislation, beyond the prosecutors and judges named explicitly here. Whether a particular officer outside the first three categories has this authority depends on whether some other separate law actually grants it to them specifically, rather than simply being assumed from their otherwise general office or position.
Why knowing who has this authority matters
A preliminary investigation conducted by someone without the authority to conduct it raises real questions about the validity of the resulting proceedings. Anyone involved in a criminal complaint — whether filing it or defending against it — benefits from confirming that the officer handling the preliminary investigation actually falls within one of these authorized categories before the process goes any further at all, and well before real time, effort, and resources are spent pursuing it.