The investigation into the British friendly-fire incident in Helmand will examine whether there was a hitch in communications between the Apache helicopter and the paratroopers.

Six of the nine injured British soldiers have returned to duty after treatment, two are still in hospital and one has been flown back to Britain in a stable condition.

Crucial to the investigation will be any messages passed between the helicopter pilot and his contact with the fighting forces on the ground, known as a joint tactical air controller, or JTAC. JTACs have become the stars of the Afghan mission, which, unexpectedly, consisted of frequent close encounters with the Taleban.

Confronted by the enemy, an army patrol sends by radio a message to command HQ calling for assistance. A British or American aircraft will be sent to the spot. The JTAC's job is to provide the co-ordinates of the Taleban so they can be targeted from the air. Sometimes the soldiers will be able to warn the helicopter of their own position, perhaps by setting off a flare.

With such close fighting there is great potential for human error resulting in what the soldiers call a “blue-on-blue”.

“Quite often there is huge pressure on the JTACs. There is pressure on them to get the co-ordinates correct under fire. They can't get one number wrong,” James Fergusson, who has interviewed many Apache pilots, said yesterday. “When the air cover arrives, the JTAC might say 'OK, can you see the rock at 11 o'clock. We are 100 yards from that'. It's high pressure stuff.”

At first there was a shortage of JTACs but the training courses have become swamped because of the urgent need for their skills. Many won medals after the British entered Helmand in 2006. Prince Harry had a similar job with his tank regiment but his role is known as a forward air controller, popularly called a “FOO”.

From the few details released, experts believe that the nine soldiers injured on Wednesday afternoon were hit with a short burst of machine-gun fire from the heavily armed helicopter. The Apaches also carry rockets and Hellfire missiles but these would be too overwhelming for such an encounter and the injuries would have been much graver. The “beaten zone”, the area of ground on which the cone of fire falls, can stretch more than 100 metres when firing from 1,000 metres away, leaving plenty of room for hitting the wrong men.

Another question will be how close the helicopter went before firing. Some say that British soldiers prefer to be helped by American pilots because they are prepared to fly lower, getting closer to the Taleban and providing more accurate support.

However, 47 civilians, most of them women and children, were killed when a US aircraft bombed a wedding party in eastern Afghanistan on Sunday, an Afghan government investigation has concluded. The nine-man investigation team found that only civilians were hit during the airstrike.

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