Police are investigating after a "suspicious item" was found at Orelia Oval before a junior football game.
Officers attended the oval around 10am on Sunday after a parent from the Kwinana Knights Junior Football Club found debris on the field.
Nearby residents reported hearing a loud bang earlier in the morning.
Police seized the item for further examination.
The investigation is ongoing and anyone with information should contact Crime Stoppers on 1800 333 000.