The jailing dissident republican Brian Shivers for the killings of sappers Patrick Azimkar, 21, from London, and Mark Quinsey, 23, from Birmingham, brings the bereaved relatives a "little bit closer to justice", a sister of one of the victims said.
Jaime Quinsey said her brother Mark's death had devastated her family.
And she appealed for the public to help catch the many others involved in the dissident republican attack.
She said her family had been handed a life sentence through the brutal loss of their loved one.
"Mark and Patrick (Azimkar) were murdered as a result of a vicious cowardly act, they were unarmed and preparing to go to serve their country in Afghanistan," she said.
"After nearly three years of heartache we have come a little bit closer to justice. One person has been convicted and another acquitted."
She appealed for the public to help police to pursue the case.
"It brings great comfort to know that this case is not closed," she added. "We know there are more people involved and we want to see them sentenced.
"My world has been torn apart. We can't change that.
"But please help the police before these people destroy more families."
Patrick Azimkar's mother Geraldine, who also spoke out after the verdict was delivered, said: "This was a terrible crime which stole Patrick and Mark's young lives from them and blighted the lives of many other people."
The bereaved mother said it was difficult to come to terms with what she described as a senseless attack.
She said: "Losing Patrick has devastated our family and has forever cast a dark shadow over our lives."
Police Service of Northern Ireland Chief Superintendent Peter Farrar welcomed the conviction.
He said police would be studying the verdict and that the investigation would continue.
"We will continue to pursue all of those involved in these evil murders," he said.
He paid tribute to the murdered men's families and pledged that the hunt to jail the rest of the republican gang will go on.
The senior officer added: "The gun attack at Massereene Barracks was wrong - pure and simple.
"It was wrong when it happened and it is wrong today. It achieved nothing. It broke lives but it did not break resolve.
"In fact it strengthened both the community resolve to reject those who use violence and the police resolve and that of our partners in this investigation to pursue all those who were involved."