A drug-using Afghan policeman turned his gun on two British service personnel in an attack prompted by losing face, an inquest has heard.
The policeman, known as Naqib, and another off-duty Afghan killed Corporal Channing Day (25), from Co Down, and Corporal David O'Connor (27) while the victims were on foot patrol on October 24 last year in Helmand's Nahr-e-Saraj district.
Cpl Day of 3 Medical Regiment was the third British servicewoman killed in Afghanistan since 2001.
The Comber-born soldier was deployed a month earlier to provide medical support to 40 Commando Royal Marines, Cpl O'Connor's unit.
The British pair were on their way to train local police in first aid and spot roadside bombs when their patrol was attacked near the village of Char Kutsa.
Their inquest at Oxford County Hall heard in the aftermath of the attack, in which Naqib was shot dead and the other policeman escaped, other Afghan policemen appeared "at ease" and "unhelpful".
Oxfordshire coroner Darren Salter, who recorded a verdict of unlawful killing while on active service, said the day before the attack Naqib and his brother lost face when they were disarmed by British soldiers outside the region's Patrol Base One.
"First of all, to the extent that there was any doubt before, this was, and should be termed as, an insider attack," Mr Salter said.
Describing Naqib, he added: "He was a known identified member of the Afghan Uniformed Police and he was the person responsible for the inside attack.
"The incident the day before when Naqib and his brother were stopped and challenged at Patrol Base One does seem to be related and does seem to be a matter that contributed."