Corporal Channing Day, 25, who served with the 3 Medical Regiment, and Corporal David O'Connor, 27, of 40 Commando, were fatally injured in an exchange of gunfire while on patrol in the Nahr-e Saraj district of Helmand province on Wednesday.
A statement from the Ministry of Defence (MOD) has confirmed that an Afghan man, believed to be a member of the Afghan Uniformed Police, also died during the incident.
He was not wearing a uniform and the UK patrol was not working with any Afghan partners at the time.
A Joint Incident Assessment Team (JIAT) has concluded that the incident was not a Blue-on-Blue (friendly fire) incident.
It is believed the deaths of Corporal Day and Corporal O'Connor were caused by a third party or parties whose identities have not been established, but are not UK personnel.
Further investigation into the involvement or otherwise of the dead Afghan male is ongoing.
The MOD said that analysis of the events surrounding the incident is continuing and is likely to take some time while forensic and other tests are carried out.
A parallel Royal Military Police investigation is also being undertaken.
Channing Amanda Day was born in Swindon, Wiltshire and grew up in Newtownards.
She joined the Army in 2005 and was deployed to Afghanistan on 2 October as a Combat Medical Technician Class One with the United Kingdom Medical Group.
She was based at Patrol Base One in the Nahr-e Saraj District of Helmand Province, where she provided medical support to 40 Commando Royal Marines.
Channing grew up into the bravest, beautiful, determined woman. She has done more in her 25 years than most women her age and we are so very proud of everything she has achieved.
In a statement by Channing's sister Lauren Day on behalf of the family, she said that they were in a state of "disbelief" but were extremely thankful for the "overwhelming support, kindness and love" received from family friends and the wider community.
"Channing knew how to light a room, so shy growing up, she didn't talk until she was four but once she started it was hard to keep her quiet," the statement read.
Lauren said that her sister always wanted to be in the army.
"She would march around the living room and never missed cadets- she loved what she did and we are so proud of her," the statement continued.
"Channing being the amazing down to earth person she was, didn't realise how beautiful she was inside and out. She was the most caring, special and supportive sister and daughter always there for us when she was needed."
The family thanked those in Afghanistan helping to take care of Channing and they extended their sympathies to the family of her colleague Corporal O'Connor's family.
"Our thoughts and prayers are with all these heroes and their families."
A book of condolence will be opened on Sunday for Corporal Day in St Mary's Church, Comber Square, which will be available to sign each day from 9am to 8pm.
The Mayor of Ards, Alderman Hamilton Gregory, said the people of Comber wished to express their deepest sympathies to the Day family as he described an "enormous sadness" in the community.
He said: "I hope, along with all our thoughts and prayers, this will be of some small comfort to the family as they grieve for their precious daughter."