Riverwood Public School teachers found a large pool of blood inside the house during a routine inspection of the school before students returned for their first day back on January 31.

Campsie commander Detective Superintendent Stephen Blackmore said at a press conference today that police had no idea what happened but the amount of blood found indicated that someone had been seriously injured.

He said police believed the injured person did not received proper medical treatment and urged anyone with information to come forward.

Checks with local hospitals have so far have failed to link any people seeking treatment to the scene.

Police said there was no evidence that the incident was connected with any staff or students at the school.

They are appealing for anyone who may have seen people in the playground of the public school in Union or Mary Street, Riverwood between January 13 to 29 to come forward.

Contact Campsie Police or Crime Stoppers on 1800 333 000.