Amanda Knox has thanked those Italians who supported her throughout her four years of prison, a day after an appeals court cleared the young American of murdering her British roommate and freed her.

Knox thanked those "who shared my suffering and helped me survive with hope," in a letter to a foundation that seeks to promote ties between Italy and the United States and which has always championed her cause.

Knox left her prison outside Perugia on Monday night, less than two hours after the verdict was read in court.

Knox was spotted in Rome's Leonardo da Vinci Airport smiling and talking with security guards who surrounded her. She embraced a man there and kissed him on the cheek.

Giulia Alagna is an acquaintance of Knox who had talked to Knox on the phone.

"She just said it was amazing to be out, a little overwhelming," said Alagna. "She sounded great, actually, a very different voice from what we heard in court because that situation was very stressful for her. She was afraid and nervous. This voice was a happy voice."

Knox and her family arrived at Heathrow Airport in London at about 5:30 a.m. and are on a British Airlines flight to Seattle. A reporter working with @KIRO7Seattle said staff on the flight had cordoned off the area where the Knox family are sitting.

A spokesman for the Knox family said they will hold a news conference at about 5:45 p.m. at the airport. Knox's parents and their legal adviser will speak to reporters, and Amanda Knox may speak. The family arrives in Seattle at 5:15 p.m.

Knox's grandmother told KIRO 7 Eyewitness News she was elated and is awaiting her family's return home.

Meanwhile, Prosecutor Giuliano Mignini expressed disbelief in the verdict, and vowed an appeal to Italy's highest criminal court.

If the highest court overturns the acquittal, prosecutors would be free to request Knox's extradition to Italy to finish the remainder of her sentence. It is up to the government to decide whether they make such a request.

Knox and her former boyfriend, Raffaele Sollecito, were convicted of sexually assaulting and murdering Kercher, who shared an apartment with Knox in Perugia. Knox was sentenced to 26 years, Sollecito to 25. Both had been in prison since Nov. 6, 2007, four days after Kercher's body was found at the apartment.

But the prosecution's case was blown apart by a court-ordered DNA review that discredited crucial genetic evidence.

The jury upheld Knox's conviction on a charge of slander for accusing bar owner Diya "Patrick" Lumumba of carrying out the killing. The judge set the sentence at three years, less than the time Knox had spent in prison.

Knox dissolved into tears as the verdict was read in a packed courtroom after 11 hours of deliberations, and she needed to be propped up by her lawyers on either side. Two hours later, she was in a dark limousine that took her out of the Capanne prison just outside Perugia, where she had spent the past four years, and headed to Rome.

"During the trip from Perugia to Rome, Amanda was serene," said Corrado Maria Daclon, the secretary general of the Italy-US Foundation, a group backing Knox, who was with her in the car. "She confirmed to me that in the future she intends to come back to our country."

While waves of relief swept through the defendants' benches in the courtroom, members of the Kercher family, who flew in for the verdict, appeared dazed and perplexed. Her sister Stephanie shed a tear, while her mother Arline looked straight ahead.

The Kerchers had pressed for the court to uphold the guilty verdicts, and resisted theories that a third man convicted in the case, Rudy Hermann Guede, had acted alone. Guede, convicted in a separate trial, is serving a 16-year sentence.