The soldiers, believed to be members of 3 Commando Brigade, were arrested in connection with an incident which occurred while they were based in Helmand last year.
The incident has been described as "an engagement with an insurgent", with no civilians involved.
During a six-month tour of duty, which lasted from April to October last year, seven servicemen from 3 Commando Brigade were killed in action, all from 42 Commando.
The tour, Operation Herrick 14, was the unit's fourth and saw the force score notable successes in capturing explosives from the Taliban.
The rules of engagement, largely derived from the Geneva Convention, dictate under what circumstances British troops are allowed to open fire, whether that is to prevent an attack by the enemy or in direct contact.