The bodies of two hedgehogs have been found inside a cardboard box where they had been deliberately set on fire.
Police were called to Mudeford Junior School in Christchurch by the fire service who had been alerted to a small blaze on the playing field.
Officers described it as an "horrific act of animal cruelty" and are checking all available CCTV for information.
Anyone with information is asked to contact Dorset Police at www.dorset.police,uk, via email email@example.com or by calling 101, quoting occurrence number 55170104072. Alternatively, contact Crimestoppers anonymously on 0800 555111 or via www.crimestoppers-uk.org.