A 32-year-old male labourer is found dead in his locked room in rural Maharashtra. The body is discovered 18 hours after death by his family. On examination, the limbs are stiff and cannot be flexed passively. The jaw is clenched, the fists are tightly closed, and the body assumes a pugilistic attitude. The ambient temperature in the room is 28°C. Which of the following statements about rigor mortis in this case is CORRECT?