PERUGIA, Italy (AP) -- Amanda Knox flew home Tuesday after four years in an Italian prison, as the dramatic reversal of her murder conviction stunned the victim's family, angered the prosecution and left questions unanswered over who killed her British roommate.

Prosecutors who saw their case collapse over discredited DNA evidence have announced they are appealing the innocent verdicts of Knox and co-defendant Raffaele Sollecito to Italy's highest court. The family of the victim, 21-year-old Meredith Kercher, said during an emotional news conference that they were back to "square one."

"If those two are not the guilty parties, then who are the guilty people?" asked Lyle Kercher, a brother of victim.

A lawyer for the sole man now convicted for the stabbing death of Kercher, Ivory Coast native Rudy Hermann Guede, said Tuesday he will seek a retrial. Prosecutors had maintained the three killed Kercher during a lurid, drug-fueled sex game.

"FREE," said local newspaper La Nazione on its front-page, dominated by a huge photo of a crying Knox, overwhelmed by emotions as the verdict was read out Monday night in a packed courtroom in Perugia.

To Knox, the verdict means freedom after four years behind bars and under the spotlight of an international press focused on her every word or gesture. The case has been a cause celebre in the U.S., and a staple of British tabloids, which took to calling her "Foxy Knoxy."

She was soon on her way home, protected by the darkened windows of a Mercedes that led her out of the Capanne prison in the middle of the night, and then Tuesday morning to Rome's Leonardo da Vinci airport, where she boarded a flight en route to the United States.

"Those who wrote, those who defended me, those who were close, those who prayed for me," Knox wrote in a letter released just hours before leaving the country, "I love you."

Knox thanked those Italians "who shared my suffering and helped me survive with hope," in a letter to the Italy-US Foundation, which seeks to promote ties between the two countries and has championed her cause.

"During the trip from Perugia to Rome, Amanda was serene," said Corrado Maria Daclon, the secretary general of the organization, who was with Knox in the car.

According to Daclon, Knox wants to come back to Italy, a country where she has become an unwilling celebrity, her face gracing the covers of gossip magazines.

But she leaves behind a country shaken by a case that raised questions over its justice system. Moments after the verdict was announced, hundreds of mostly university-age youths gathered in the piazza outside the courtroom and jeered at the news. "Shame! Shame!" and "Murderers!" they yelled.

Prosecutor Giuliano Mignini expressed disbelief at the verdict and said he will appeal to Italy's highest criminal court after receiving the reasoning behind the acquittals, due within 90 days.

"Let's wait and we will see who was right. The first court or the appeal court," Mignini told The Associated Press on Tuesday. "This trial was done under unacceptable media pressure."

Mignini noted that 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.

"There is a heavy conviction for slander. Why did she accuse him? We don't know," said Mignini.

The highest court's remit is to rule on whether any procedures had been violated, and the hearing generally takes one day in Rome. Defendants are not required to attend.

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

Mignini has been conducting the investigation from the start, and said he never had any doubts that the defendants were guilty. In 2009 he won murder convictions for the two and heavy sentences: 26 years to Knox and 25 to Sollecito. But the prosecution's case was blown apart by a DNA review ordered during the appeals trial that discredited crucial genetic evidence.

Prosecutors maintain that Knox's DNA was found on the handle of a kitchen knife believed to be the murder weapon, and that Kercher's DNA was found on the blade. They said Sollecito's DNA was on the clasp of Kercher's bra as part of a mix of evidence that also included the victim's genetic profile.

But the independent review - ordered at the request of the defense, which had always disputed those findings - reached a different conclusion.

The two experts found that police conducting the investigation had made glaring errors in evidence-collecting and that below-standard testing and possible contamination raised doubts over the attribution of DNA traces, both on the blade and on the bra clasp, which was collected from the crime scene 46 days after the murder.

The review was crucial in the case because no motive has emerged and witness testimony was contradictory. Mignini's description of Knox as a manipulative liar also failed to sway the eight-member jury.

Minigni has run into controversy in other cases. He was convicted in 2010 of abusing his office in an another case, by trying to influence officials investigating the 1985 death of a doctor thought to be involved in a Satanic group. Mignini, who has denied wrongdoing and appealed the decision, said at the time that the ruling would not impact the Knox and Sollecito prosecution, which was then under way.

One of the biggest questions left unanswered centers on Guede, a small-time drug dealer and drifter who spent most of his life in Italy after arriving in Italy from Ivory Coast. Guede used to play basketball near the crime scene and was a passing acquaintance of Knox. Sollecito says he did not know him.

The courts that convicted him say Guede took part in the sexual assault that led to Kercher's stabbing death, leaving traces of DNA on the victim and at the crime scene. Guede was convicted in a separate fast-track procedure and saw his sentence cut to 16 years in his final appeal.

Defense lawyers maintain that Guede was the sole killer, while prosecutors say that bruises and a lack of defensive wounds on Kercher's body prove that there was more than one aggressor holding her into submission. However, they could never quite explain how Knox and Sollecito, who had been dating for less than a week, would be be involved in an extreme sexual scenario with somebody only one of them barely knew.

The highest court, in upholding his conviction, said Guede had not acted alone. However, the court's ruling does not name Knox and Sollecito as Guede's accomplices, saying it was not up to the court to determine that.

But Kercher family was perplexed, saying, in Lyle Kercher's words, that the verdicts "obviously raises further questions in as much as there is a third defendant, Rudy Guede, who is convict... As far as I understand, the courts agree he wasn't acting alone."

Guede says he is innocent, though he admits being in the house the night of the murder. Taking the stand during the appeals trial, he said he believes Knox and Sollecito are guilty.

Guede lawyer Valter Biscotti told The Associated Press on Tuesday he would seek a revision of the trial for his client. He refused any further comment on the verdicts.

The case captivated Italy and is likely to remain one of several unsolved judicial mysteries in the country.

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