The senior detective in charge of the Angela Wrightson case has said it is among the most disturbing of his career.
Police have also issued troubling images of the murder weapons used in the brutal attack on Ms Wrightson.
In a statement, Detective Chief Supt Peter McPhillips said: "This was a highly unusual and shocking incident.
"Throughout almost 25 years of service I have never come across such a brutal murder committed by such young girls.
"Angela was subjected to a prolonged, sustained attack and t he pathologist identified a significant number of injuries to her body caused by at least twenty-five blows with weapons.
"Officers have worked closely with the Crown Prosecution Service and other partner agencies to build a case against the two girls and I am pleased th at the overwhelming weight of evidence in the prosecution case has resulted in guilty verdicts today.
"In my experience this is an almost unique investigation in terms of the age of the offenders and the fact that they are girls.
"Many questions remain unanswered about the motive for the murder but the family of Angela who have had to endure the most shocking and traumatic details unfolding over the last few weeks will get some satisfaction from knowing that her killer s have now been convicted of the killing.
"The Local Safeguarding Adult and Children Board have commissi oned a Safeguarding Adult Review and a Serious Case Review, which hopefull y will examine the circumstances leading up to the murder. The reviews will determine whether any lessons can be learned and the report will be made public."