Image caption Elizabeth Edwards, 49, and her daughter Katie, 13, were found dead at Dawson Avenue in Spalding in April

A 15-year-old girl has been found guilty of murdering a mother and daughter.

The teenager, who cannot be named, was 14 when she and her boyfriend plotted to kill Elizabeth Edwards, 49, and her daughter, Katie, 13, as they slept at their home in Spalding, Lincolnshire.

Jurors heard the victims were smothered with pillows and repeatedly stabbed in a "cold, calculated and callous" act.

A 15-year-old boy admitted murder at the start of the trial.

The girl had denied murder but admitted manslaughter on the grounds of diminished responsibility.

What drove teen sweethearts to murder?

Image copyright Police issue Image caption Police have released a picture of the knife used in the murders

During the six-day trial, Nottingham Crown Court heard the love-struck teenagers' relationship compared to that of US gangsters Bonnie and Clyde.

Jurors had heard how the teenagers meticulously planned the murders before carrying them out "to the letter".

The seven men and five women of the jury took just two and a half hours to reach their verdicts.

More on this and other stories from across Lincolnshire

Media playback is unsupported on your device Media caption Jane Blandford, a friend of Liza Edwards, said those who killed her were 'scum'

The bodies of Mrs Edwards and her daughter were found in their beds on 15 April.

Mrs Edwards was stabbed by the boy eight times, twice in the throat in a deliberate attempt to ensure her daughter was not woken by screams or cries for help. One blow injured her jugular and the other almost completely severed her windpipe.

Katie Edwards was stabbed twice in the neck with severe force before she was smothered by the boy.

The girlfriend, who did not physically take part in the killings, told police she heard Mrs Edwards "struggling and gurgling" and said she heard Katie Edwards say "I can't breathe" as she was set upon.

Image copyright PA Image caption Flowers and tributes were laid outside the property where the bodies of the mother and daughter were found

'Felt like murdering'

Following the killings the couple shared a bath, had sex and watched four Twilight movies before they were arrested.

The court heard the teenagers had planned to commit suicide after the killings but decided not to go through with that part of their plan.

The knife used in the murders - a black-handled kitchen knife with a 20cm blade - was found discarded in Katie's bedroom.

Prosecutor Peter Joyce QC described the murders as "cold, calculated and callous".

Image copyright SWNS Image caption Police described the murder of Elizabeth and Katie Edwards as a horrendous crime

In her police interview the girl told officers she had "felt like murdering for quite a while".

She told detectives she and her boyfriend formed their plan on 9 April, four days before the killings, and once the mother and daughter were dead she "felt fine".

The court heard she told police interviewers: "I was OK with it. Just the fact that it happened so quickly gave me peace of mind."

In court: BBC correspondent Danny Savage

Image copyright PA

These two teenagers now have the notoriety of becoming the youngest couple in Britain to have killed. It was a planned double murder born out of an intense relationship. They were two 14-year-olds who in the court appearances following their arrest exchanged smiles and loving looks.

But when the boy pleaded guilty to murder last week, the body language between the two was very different. No furtive looks, no smiles. They coldly ignored each other. It emerged over the coming days they had split up. Hardly surprising as they've both been kept in separate secure accommodation.

What ruins they have left behind though. Drawn together by suicidal thoughts, they convinced each other over a McDonald's meal that the solution to the grudge she had against 49-year-old Liz Edwards was to kill. Intense planning followed where the murder of Ms Edwards' daughter was also approved.

The boyfriend wounded the school dinner lady terribly before smothering her with a pillow. It wasn't quick - she struggled and fought for her life while the girl watched on.

The girlfriend had reservations about creeping into Katie Edwards' smaller room with its bunk bed and killing her too. Those concerns weren't strong enough to stop the boy. The account of how he murdered the 13-year-old are some of the most harrowing details I have heard in 17 years of court reporting.

Their crimes of passion are truly shocking. It's worth pausing for a moment to reflect on what two children did to another child and questioning just how their relationship got to the stage where their murderous plan seemed perfectly plausible.

'Senseless and unprovoked'

In the girlfriend's defence, psychiatrist Dr Indranil Chakrabarti said he believed the teenager, who did not give evidence, had been suffering from an adjustment disorder at the time of the killings.

He said as a result she was unable to form a rational judgment and unable to exercise self control.

But consultant forensic psychiatrist Dr Philip Joseph disagreed with that diagnosis. He stated that even if the girl had been suffering from such a disorder "it is blindingly obvious that it has got nothing to do with the killings".

He told the court how during his interview with the girl in September she was "inappropriately upbeat" and did not look upset or show remorse.

Comparing the teenagers to American crime duo Bonnie Parker and Clyde Barrow, Dr Joseph said: "If they hadn't got together and had the intense, toxic relationship, they [the killings] would never have happened."

As the verdicts were read out the girl showed little emotion, but a number of people in the public gallery cried, sobbed and gasped.

The judge, Mr Justice Haddon-Cave, paid tribute to the detectives involved in investigating what he described as an "exceptional case by any standards".

He adjourned sentence on both defendants until November and will also rule on whether they should be identified at a later hearing.

Image copyright PA Image caption Police described it as a difficult and unprecedented case for all involved

Speaking outside court, Det Ch Insp Martin Holvey said: "This case was distressing for the families involved, for Katie's school friends and for Elizabeth's colleagues, friends and the children at the school where she worked as well as having a shocking impact on the wider community of Spalding.

"What makes this case even more shocking is that these two were just 14 when they planned and committed these callous, senseless and unprovoked attacks.

"This case has left a number of lives in ruins not only Elizabeth and Katie's families but also the two juveniles who committed this horrendous crime."