Final answer:
EMTs should take over from a police officer after the shock has been delivered or a "no shock indicated" message is received from the AED, to ensure effective and timely treatment.
Step-by-step explanation:
The point at which EMTs should take over from a police officer administering an AED to a cardiac arrest victim is after the shock has been delivered or a "no shock indicated" message is received. This allows the defibrillation process to be completed without interruption, providing the patient with a structured and effective response. Automated external defibrillators (AED) are designed to be used by laypersons and contain verbal instructions for operation. They work by analyzing the heart's rhythm and, if appropriate, automatically delivering a shock to treat ventricular fibrillation. Following AED guidelines and ensuring prompt, continuous care are crucial in these situations.