The two senior officers were thousands of miles from the dust and danger of Helmand province in Afghanistan.

One had recently returned from the war where his troops reported their understanding that a policy of execution-style killings was being carried out by Special Forces.

The other had been at headquarters, reading reports from the frontline with growing concern. They showed a sharp rise in the number of "enemies killed in action" (EKIA) by UK Special Forces.

Special Forces are the UK's elite specialist troops, encompassing both the SAS (Special Air Service) and the SBS (Special Boat Service).

After the conversation, a briefing note believed to have been written by one of the most senior members of UK Special Forces was passed up the chain of command.

The message contained clear warnings for the highest levels of Special Forces and concluded that these "concerning" allegations merit "deeper investigation" to "at worst case put a stop to criminal behaviour".

The documents were released to solicitors Leigh Day, as part of an ongoing case at the High Court, which will rule on whether allegations of unlawful killing by UK Special Forces were investigated properly.

The man bringing the case is Saifullah Ghareb Yar. He says that four members of his family were assassinated in the early hours of 16 February 2011.

It follows a BBC Panorama programme last year, which reported on the deaths. The programme worked with the Sunday Times Insight team to reveal evidence of a pattern of illegal killings by UK Special Forces.

The government maintains that the four members of Saifullah's family were killed in self defence.

But now correspondence in the newly-released documents shows that some had grave concerns about the UK Special Forces mission.

Just hours after the elite troops had returned to base, other British soldiers were exchanging emails describing the events of that night as the "latest massacre".

'Shaking with fear'

At 01:00 in Nawa, rural Helmand, on 16 February 2011, Saifullah's family were asleep in their home.

They woke suddenly to the sound of helicopter rotors, followed by shouting through megaphones. Saifullah was still a teenager but he was about to find himself in the middle of a Special Forces "kill or capture" mission.

These "night raids" were a common tactic at the time. They were typically carried out in partnership with Afghan forces under cover of darkness. Their purpose was to target senior members of the Taliban.

"My whole body was shaking because of the fear. Everyone was frightened. All the women and children were crying and screaming," Saifullah told BBC Panorama.

He described how his hands were tied and he was put in a holding area with the women and children. He had not been there for long when he heard gunfire.

After the troops had left, the bodies of his two brothers were discovered in the fields surrounding their home. His cousin had been shot dead in a neighbouring building.

Going back into his house, Saifullah found his father, lying face down on the ground.

"His head, the forehead area, was shot with many bullets, and his leg was completely broken by the bullets," he said.

Last year, Panorama exposed how the intelligence that identified the targets for these raids was often deficient.

Philip Alston, the former UN Special Rapporteur on executions, told the programme: "I have no doubt that overall many of the allegations [of innocent people being killed] are justified, and that we can conclude that a large number of civilians were killed in night raids, totally unjustifiably."

Saifullah believes his family were wrongly targeted and then executed in cold blood.

In Nawa district, there was an outcry after the killings. The Governor of Helmand believed the victims were innocent civilians.

British military emails from the aftermath of the raid obtained by Panorama suggest that eyewitnesses from the Afghan military supported Saifullah's version of events.

A commanding officer from the Afghan forces is quoted as having said that no one was firing at the British but the four family members were shot anyway and that "he sees this as confirmation that innocents were killed".

The Afghan commander suggests that "two men were shot trying to run away, and that the other two men were "assassinated" on target after they had already been detained and searched".

The correspondence shows that these events sent shockwaves through the British military from Helmand to London.

Emails outline concerns over Afghan forces refusing to accompany the British on night raids because they did not believe the killings were justified. This was not the first time that the Afghan forces had made this complaint.

One senior Special Forces officer comments that this kind of falling out "puts at risk the [redacted] transition plan and more importantly the prospects of enduring UK influence" in Afghanistan.

"Aside from alienating our Afghan allies, the narrative of murderous British forces played right into the hands of the insurgents," said Frank Ledwidge, a former military intelligence officer who served as a justice adviser in Helmand.

"The actions of some Special Forces actively undermined the overall counterinsurgency mission, which was challenging enough already," he said.

'You couldn't make it up'

Among the documents released to the court is a detailed summary marked "secret".

It includes an extract of the classified operational summary (OPSUM), which provides the official account of what the strike team did at Saifullah's home.

The UK Special Forces reported that after initially securing the compound they went back in to search the rooms with one of the Afghan men they had detained.

While there, it says he suddenly reached for a grenade behind a curtain.

"He poses an immediate threat to life and is engaged with aimed shots. The assault team members take cover. The grenade malfunctions and does not detonate," the OPSUM says.

That man was Saifullah's father.

After the shooting, the OPSUM reports that another Afghan was moved into the neighbouring compound to help with the search of the buildings. They say he was also shot after picking up a weapon.

That man was Saifullah's cousin.

Both of Saifullah's brothers are reported to have run away when they spotted the unit arriving. One hid in a bush with a grenade and was shot and killed when the explosive was spotted, says the OPSUM.

The other was reported to be hiding a short distance away with a machine gun. When he emerged from a hiding place under a blanket with the weapon, he too was shot.

This official account of the killings was met with suspicion by some in the British military.

An internal email requests a copy of the OPSUM within hours of the killings and asks: "Is this about [redacted] latest massacre!"

The reply includes a summary of the unlikely events in the official report and concludes by saying: "You couldn't MAKE IT UP!"

It looks as if the soldiers reading these reports had concerns that they were being falsified using near-identical cover stories.

A spokesperson for the Ministry of Defence said "This is not new evidence, and this historical case has already been independently investigated by the Royal Military Police as part of Operation Northmoor. It has also been subject to four reviews conducted by an Independent Review Team.

"These documents were considered as part of the independent investigations, which concluded there was insufficient evidence to refer the case for prosecution.

"The Service Police and the Service Prosecuting Authority of course remain open to considering allegations should new evidence, intelligence or information come to light."

'Perverting the course of justice'

The suspicious pattern of similar incidents leading to the killing of Afghan men during Special Forces night raids caught the eye of several people back at UK Special Forces headquarters in England.

The court documents show a review was ordered.

A Special Forces Major examined all of the official reports of killings by the elite troops between December 2010 and April 2011.

He wrote to other senior officers to say the number of killings led him to conclude "we are getting some things wrong, right now".

His report highlighted 10 incidents in which the similarity of the accounts in official paperwork raised his suspicions.

All involved the shooting of men who were detained before they unexpectedly grabbed a weapon during a search of the buildings.

The Major also found at least five separate incidents where more people were killed than there were weapons recovered. That means either the weapons went missing or the people who were killed were not armed.

In one case, nine people had been killed and only three weapons had been recovered.

The newly-released evidence appears to support revelations in last year's Panorama and Sunday Times investigation.

Panorama reported that a large scale Royal Military Police (RMP) investigation called Operation Northmoor had linked dozens of suspicious killings on night raids. Among them were the deaths of Saifullah's family members.

When the RMP interviewed the Special Forces troops who took part in the raid of 16 February 2011, all of them claimed they could not remember the specifics of the mission that night.

Operation Northmoor was investigating whether official operation reports had been falsified. In one case, the RMP had even brought charges against members of the UK Special Forces for murder, falsifying a report and perverting the course of justice.

But the charges were dropped and the government closed down Operation Northmoor without prosecuting a single case. Insiders said it was closed too soon for them to complete their investigation.

"It seems to be one of the unique characteristics of British Special Forces that they are truly accountable to no-one," said Frank Ledwidge.

"Accountability must apply to everyone and particularly to the senior commanders and politicians who have allowed, condoned or ignored these alleged crimes and created the environment for them to happen".