Amanda Knox leaves Italy after murder acquittal

American student Amanda Knox headed for home on Tuesday after spending four years in an Italian jail, leaving the family of Meredith Kercher to express their anguish at being no closer to the truth about her murder.

Knox left Rome shortly before midday for London where she and her family were due to board a connecting flight to their home in Seattle, airport officials said.

The 24-year-old had broken down in sobs on Monday after an appeals court in Perugia ruled she and her former boyfriend, Italian computer student Raffaele Sollecito, should be freed immediately.

Prosecutors said on Tuesday they would appeal against the ruling and Kercher's disappointed family said the search for who killed the British student in 2007 would go on.

"We're still absorbing it. You think you've come to a decision and now it's been overturned," Meredith's morther Arline told reporters at a news conference.

The prosecution will now appeal to the Court of Cassation, Italy's highest appeals court.

Kercher's sister Stephanie said they would wait for the written explanation of the acquittal verdict in the hope that all the killers would eventually be found.

"Once we've got the reasons behind the decisions for this one, then we can understand why they have been acquitted of it and work towards finding those who are responsible," she said.

"That's the biggest disappointment -- not knowing still and knowing that there is someone or people out there who have done this."

Ivorian drug dealer Rudy Guede is serving a 16-year sentence for his role in the murder. But investigators believe more than one person held Kercher down while she was stabbed and had her throat cut.

Knox and Sollecito had appealed against a 2009 verdict that found them guilty of murdering 21-year-old Kercher during what prosecutors had said was a drug-fuelled sexual assault.

Knox has not spoken in public since but she thanked her supporters in a letter to an Italian-American foundation that was published by Italian news agency Ansa.

British Prime Minister David Cameron urged people not to forget Kercher's parents.

"Those parents ... had an explanation for what had happened to their wonderful daughter, and that explanation isn't there ... I think everyone today should be thinking about them and how they feel," he told ITV.

Outside the court on Monday, hundreds of people whistled, booed and shouted "shame, shame" and "bastards" at the courtroom and at US television crews.

The appeal trial gripped attention on both sides of the Atlantic. There was an outpouring of sympathy and outrage from many in the United States who regarded Knox as an innocent girl in the clutches of a medieval justice system.

But with another appeal, Kercher's family said the lack of clarity made it hard for them to come to terms with the killing.

"It's still very difficult to speak in terms of forgiveness until we know the truth," said Meredith's sister Stephanie. "Until the truth comes out we can't forgive anyone because no one's admitted to the crime.

"It may be a case of waiting years to get to the truth, we just have to wait again now."

The verdict was an embarrassment for the prosecutor and Italian police investigators. Independent forensic investigators sharply criticised scientific evidence in the original investigation, saying it was unreliable.

Kercher's half-naked body, with more than 40 wounds, was found in 2007 in the apartment she shared with Knox in the Umbrian hill town of Perugia where both were studying.

Knox and Sollecito, 27, said they were innocent.

Sollecito, who had been held in a separate jail near Perugia, also left custody immediaterly after the verdict.

The court upheld a conviction against Knox for slander after she had falsely accused barman Patrick Lumumba of the murder. It sentenced her to three years in prison, a sentence which she has now served.

Kercher, a Leeds University student, was on a year-long exchange programme in Perugia. Her murder brought a flood of unwelcome attention to the medieval town in central Italy that her family said she loved.

The murder investigation showed she was pinned down and stabbed to death and evidence suggests Guede did not act alone.

Prosecutors had said Kercher resisted attempts by Knox, Sollecito and Guede to involve her in an orgy. Their case was weakened by forensic experts who dismissed police evidence that traces of DNA belonging to Knox and Kercher were found on a kitchen knife identified as the murder weapon.

The experts also said alleged traces of Sollecito's DNA on the Briton's bra clasp may have been contaminated.

The defence argued that no clear motive or evidence linking the defendants to the crime had emerged, and said Knox was falsely implicated in the murder by prosecutors determined to convict her regardless of the evidence.