Footage discovered on a serviceman's laptop prompted the arrest of seven Royal Marines on suspicion of murder over an incident in Afghanistan, Sky News has learned.
The Ministry of Defence (MoD) said the arrests by the Royal Military Police relate to an incident that happened after an engagement with an insurgent.
Sky sources revealed it only came to light following an arrest last week by civilian police - for a separate matter - of a man who had been serving in Afghanistan.
During that investigation, they had to look at his laptop - where they discovered a video that showed what were allegedly Royal Marines in a compound in Helmand Province with what appears to be an injured Taliban insurgent.
Sources say the clip contains a conversation about what to do with the injured man and whether to administer first aid.
The clip allegedly then ends and the fate of the wounded fighter is unclear.
No civilians were involved in the incident.
Sky sources believe the men arrested were serving at the time of the alleged incident with 42 Commando Royal Marines, based at Bickleigh, near Plymouth in Devon, between April and June last year.
The arrests are thought to have been made at around 4.30pm on Thursday.
The MoD said: "The investigation will now be taken forward and dealt with by the Service Justice system.
"These arrests demonstrate the Department and the Armed Forces' determination to ensure UK personnel act in accordance with their Rules of Engagement and our standards.
"It would be inappropriate to make any further comment while the investigation is under way."
The rules of engagement, largely derived from the Geneva Convention, dictate under what circumstances British troops are allowed to open fire in direct contact or to prevent attacks.
Shashank Joshi, a defence expert at the Royal United Services Institute , told Sky News it was significant that the MoD had stated no civilians were involved.
"(This) suggests one possibility may have been rules of engagement surrounding the surrender of insurgents were not obeyed," he said.
"If an insurgent surrenders or makes clear his wish to surrender and is subsequently dealt with as if he were in combat or is summarily shot, that would constitute a very severe breach of the rules of engagement."
He added: "Soldiers have to make split-second decisions, not only about whether an individual is an insurgent or a civilian, but also, if they are an insurgent, what level of threat they pose.
"I think the categorical nature of this statement suggests the MoD is fairly confident here it wasn't such a grey area. That's my reading of their statement, that actually this may be a fairly clear-cut case."
During a six-month tour of duty starting in April last year, 3 Commando Brigade saw seven servicemen 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.
Brigadier Ed Davis, who commanded Task Force Helmand and led 3 Commando Brigade during the tour, said his men had "interdicted" over 2.5 tons of home-made explosive by August last year, leading to a 43% drop in enemy attacks.
Sky's Foreign Affairs Editor Tim Marshall said: "It's looking to me like a rules of engagement issue and that is very, very complicated. The rules of engagement are not widely known, because they don't want the enemy to know them."
The arrests are thought to be the first time UK servicemen have been held on suspicion of such charges during the conflict in Afghanistan.
The Afghan authorities have welcomed the investigation.