The Samoa police professional standards unit wants to know how police got to use an illegal assault rifle.
The AK-47 weapon was handed in to police by the customs to be destroyed during an arms amnesty, but instead it had been kept in the amoury and used during police raids.
The acting Police Commissioner, Afamasaga Michael So'onalole, said he had ordered the investigation after complaints from the public and media inquiries after allegations of the weapon being carried by a senior officer during raids.
He said the registration of the unlawful gun under the name of the police ministry was thought to have been ordered by the suspended police commissioner.
Afamasaga said the investigation could lead to charges being laid.