A 15-year-old girl is facing a life sentence after being found guilty of murdering a mother and daughter who were stabbed through the throat in a "senseless and unprovoked" attack.
The teenager, who cannot be named because of her age, had claimed a mental disorder diminished her responsibility for the killings of dinner lady Elizabeth Edwards and 13-year-old Katie Edwards.
But a five-day trial at Nottingham Crown Court heard that the schoolgirl was not suffering from a mental illness when she and her boyfriend planned the killings "to the letter" when they were both aged 14.
The girl, who remained composed in the dock, and her boyfriend, also now 15, are thought to be the youngest defendants to be convicted of double murder in a British court.
After the verdicts had been delivered, Mr Justice Haddon-Cave said it had been an "exceptional case by any standards" and praised the quiet dignity of family members who sat through the trial.
Jurors were told that the sweethearts - also believed to be the youngest couple to be convicted of murder - shared a bath, had sex, and watched four vampire-themed Twilight films after the killings in Spalding, Lincolnshire, last April.
The female defendant admitted manslaughter but denied murder, claiming to be suffering an abnormality of mental function which impaired her ability to form rational judgments.
But jurors - who heard that the defendants' toxic Bonnie and Clyde-style relationship "led on to" the killings - took two-and-a-half hours to reject the girl's defence and convict her of murder.
Her boyfriend, who stabbed and smothered the victims as they slept, pleaded guilty to murder before the trial began.
At the start of the hearing, prosecutor Peter Joyce QC said the victims were stabbed a total of 10 times in a "cold, calculated and callous" pre-planned attack at their home.
In police interviews a day after her arrest, the girl said her boyfriend had knifed Ms Edwards through the voice box to ensure her daughter was not woken by screams or cries for help.
Police found the bodies of Ms Edwards and her daughter on Friday April 15 this year when three officers forced their way into their house in Dawson Avenue, Spalding.
Post-mortem examinations found signs of defensive injuries on the older victim's hands, suggesting she had tried to fight off the boy, while her daughter had been stabbed twice in the neck.
In police interviews and assessments with psychiatrists, the female defendant described the killings as "a breeze" and gave a horrifying account of blood being spattered on a wall and her boyfriend's face and hands.
In lengthy admissions to police, the schoolgirl said she and her boyfriend had planned the killings after a conversation which began with with a joke suggestion of committing a murder.
Explaining that failed attempts to put their scheme into action had taken place on the previous two days, the girl told officers: "We went over the plan over and over again."
After the killings, the girl said, her boyfriend "seemed fine" and they took a bath for around 20 minutes, opting not to follow their plan and take their own lives.
Consultant forensic psychiatrist Dr Philip Joseph told the trial he believed the couple's "intense, toxic" relationship was the primary reason for the killings.
He told the jury: "A group dynamic can lead you to a course of action you would never have contemplated on your own.
"Bonnie and Clyde ... that sort of intense attraction, emotional closeness - them against the world. It's that sort of thing that led on to this."
Both killers face indefinite detention at Her Majesty's pleasure - the juvenile equivalent of a life term for an adult - and will be sentenced in November.
Speaking to the media outside court, Detective Chief Inspector Martin Holvey, of East Midlands Major Crime Unit, said: "This tragedy was distressing for all families involved, for Katie's school friends, for Elizabeth's colleagues, friends and children at the school where she worked, as well as having a shocking impact on the wider community of Spalding.
"Today the court has found a 15-year-old guilty of both murders. A teenage boy has also pleaded guilty.
"What makes this case even more shocking is that these two were 14 years of age when they planned and committed these callous, senseless and unprovoked attacks on Elizabeth and Katie.
"This case has left a number of lives in ruins, not only Elizabeth and Katie's family, but also the two juveniles who committed these horrendous crimes."