Not all London Metropolitan Police carry firearms. Those who do have volunteered for special training. Authorized firearms officers (AFOs) do not use guns in the normal course of their duties but are available to assist if needed. Others, the Specialist Firearms Officers (SFOs), serve in the Specialist Firearms Command, the British equivalent of the American SWAT teams. Many served previously in the Royal Army, Royal Marines, Special Air Service, or, in the case of The Witness character Sergeant Casey, the Special Boat Service. All receive additional training every six weeks to keep their skills at the highest level.