Royal Marine accused of executing Taliban prisoner 'made a dreadful mistake' Commando identified as 'Marine A' shot captured Afghan fighter in the chest The 39-year-old said he thought the insurgent was already dead when he shot him at close range The incident was filmed by a camera mounted on the helmet of a second man, 'Marine B' By Ian Drury, Defence Correspondent A Royal Marine accused of executing a severely-wounded Taliban prisoner made a "dreadful mistake in a corner of a foreign field," a court martial heard. The 39-year-old commando, who can only be identified as Marine A, shot the captured Afghan fighter in the chest at close range with a 9mm pistol, the trial was told. The experienced sergeant, a 15-year veteran who served in Northern Ireland, Iraq and Afghanistan, claims he thought the insurgent was already dead. Marine A is on trial with two comrades, identified as Marine B and C, who allegedly helped and encouraged the killing of the detainee during a patrol in Helmand. Seconds after the pulling the trigger he told comrades he had "broken the Geneva Convention" - but insists the remark referred to him desecrating a corpse, not killing an injured detainee. In a closing statement to the court martial Anthony Berry QC, defending Marine A, said the serviceman was "fine, battle-hardened, tough and devoted" with an "unblemished character." Mr Berry said: "If he made a dreadful mistake in a corner of a foreign field while commanding his men it was not, we suggest, murder he committed. He thought the Taliban was dead when he discharged his pistol and that is the end of the matter. Marine A is on trial with two comrades, identified as Marine B and C, who allegedly helped and encouraged the killing of the detainee during a patrol in Helmand on September 15, 2011. All three deny murder. Film footage of the shooting, captured on a camera mounted on the helmet of Marine B, has been played to the hearing in Bulford, Wiltshire. The seven-strong court martial board has been told that Marine A's unit, part of 42 Commando, went on patrol almost every single day during its six-month deployment in the warzone. Up to 20 comrades had already been killed or maimed by Taliban bombs and bullets by the day of the incident. Some victims" blown off limbs had been hung in trees by insurgents as "trophies," the court heard. The victim was found with an AK47 assault rifle and grenade after an Apache helicopter blitzed him with cannon fire following an attack on a British base. The court martial in Bulford, Wiltshire, continues The suspects allegedly dragged the terribly wounded insurgent out of view of a British surveillance balloon and the circling gunship before Marine A shot him. All three later claimed the Apache caused his fatal injuries. Mr Berry said Marine A had fired a bullet into the apparently dead insurgent because of a "lapse of control" in a stressful deployment. He said the unit, based at Command Post Omar in Nad-e Ali, a Taliban stronghold, was operating in an "inhospitable environment with a wicked and brutal enemy." You might at any moment step on a mine, you might be shot at, blown up in a car or blasted by a grenade... It is hard to expect soldiers in these circumstances to act with a degree of circumspection. Marine B, a 31-year-old university graduate, has been accused of plotting to kill the severely-injured man, pretending to give him first aid while watching for the helicopter has left the scene so the "coast was clear." But his barrister Martin Meeke QC said prosecutors had "failed to find the evidence" that the commando was allegedly part of a murder plan. He said: "It is not an offence to be present when an offence is committed, nor of standing by and not taking steps after a crime is committed." Mr Meeke said if Marine B had been aware that the insurgent would be shot, he would not have continued to film the alleged slaying on his helmet-mounted camera. "If he knew what was going to go on would he have allowed it to go on recording, or would we have had him saying again, "Let's switch this f***** off"?" The court martial continues.