Tune in to "AC360º" at 8 p.m. ET Tuesday as Anderson Cooper gets worldwide reaction to Amanda Knox's release and her return to the U.S.
Perugia, Italy (CNN) -- Murder victim Meredith Kercher's family cannot forgive anyone until they know the truth about her killing, Kercher's sister said Tuesday as Amanda Knox flew out of Italy, a free woman after four years.
Knox and her Italian ex-boyfriend Raffaele Sollecito were dramatically cleared of the murder Monday, in a jury ruling that left Knox sobbing and almost unable to walk out of the court unaided.
The decision stunned the Kercher family, who were visibly downcast Tuesday morning.
It sends them "back to square one," Kercher's brother Lyle said.
Stephanie Kercher, the victim's sister, demurred when asked if the family is ready to forgive Meredith's killer or killers.
"Until the truth comes out, we can't forgive anyone," she said.
She said the family looked forward to reading the judge's explanation of how the jury reached its decision, which by law must be published within 90 days.
Not knowing why one jury two years ago convicted Knox and Sollecito of murder, sexual assault and other crimes related to Kercher's killing, while another said Monday they are innocent is "the greatest disappointment," Stephanie Kercher said.
Kercher's mother, meanwhile, did not speak ill of Knox, even when asked what she thought of the American's assertion that her life had been destroyed by the ordeal.
"That's probably true in a way," Arline Kercher said.
In an interview with CNN, Lyle Kercher said the family accepts the decision, as it accepted the decision in the original trial. "We have the utmost respect, obviously, for the decision and the integrity of the court," he said, adding, "We will abide by that."
Knox, who arrived in Seattle to cheers just after 8 p.m. ET, thanked her supporters and those who helped her family in emotional remarks at a brief news conference.
"I'm really overwhelmed right now," she said. "I was looking down from the airplane and it seemed like everything wasn't real."
She told her supporters, "Thank you for being there for me."
Knox spent Monday night with her mother before leaving Italy, her lawyer Carlo Dalla Vedova told CNN.
Prosecutor Giuliano Mignini said Tuesday that he will appeal the ruling to Italy's High Court.
When the appeal reaches the High Court, the hearing will be very short and dependent on key technical issues and arguments, not a review of the actual trial itself, according to Luiss University law professor Nicola Di Mario.
Kercher's family said it would support such an appeal, but added it was the state's decision.
If Italy's highest court overturns the lower court's decision, it effectively revalidates the trial court's sentence, which means 26 years in prison for Knox. Italy would then have to put in a request to U.S. authorities for extradition, and it would remain to be seen if that would be granted. If the court does not overturn the appeals ruling, Knox could not be tried again for the same crime under the "double jeopardy" rule.
Knox and Kercher were students at Perugia's university for foreign students when Kercher's partly clothed body was found stabbed and with her throat slashed in the house they shared.
Monday's ruling prompted a wave of euphoria by Knox, her family and supporters, as well as howls of disapproval outside the courtroom.
"Perugia is a quiet town. The fact that people came and cry out loud 'shame' on the ruling means something," Mignini said.
After the ruling, Knox broke down sobbing and was escorted out of the courtroom by two people.
"We're thankful that Amanda's nightmare is over," Knox's sister, Deanna, said on the courthouse steps. "She suffered for four years for a crime she did not commit."
Knox and Sollecito were convicted in 2009 of murder, sexual assault, possession of a weapon, interfering with a crime and theft. The jury cleared Knox and Sollecito of those charges, freeing them from their respective 26- and 25-year sentences.
But the jury Monday upheld Knox's conviction on the charge of defamation against Patrick Lumumba, an early suspect in the case whom Knox accused of killing Kercher.
Lumumba was arrested, but released after his alibi checked out. He later sued Knox, winning 40,000 euros ($54,000) in damages. Knox was sentenced Monday to three years on the defamation charge, but received credit for the years she has already spent behind bars, her attorney said.
A third person, drifter Rudy Guede, was convicted separately of involvement in the killing and is serving 16 years. Defense teams for Knox and Sollecito have suggested Guede could have been the sole killer.
The jury evidently believed Knox's impassioned final statement to the court, delivered in a voice trembling with emotion.
"I am not what they say I am -- perverse, violent. ... I haven't murdered. I haven't raped. I haven't stolen," Knox said in the most important speech of her life.
Sollecito put his claim simply in his own closing statement.
"I have never hurt anybody," he said.
As he concluded, he dramatically removed his plastic "Free Amanda and Raffaele" bracelet, saying: "I have never taken it off since it was given to me. ... I think now is the moment to take it off."
Following the rulings, Knox returned briefly to the prison and was greeted by cheers and shouts of "well done," said Rocco Girlanda, a member of the Italian parliament who became an advocate for the 24-year-old American.
"Her first desire is to lie down on a green field," he said
CNN's Hada Messia, Antonia Mortensen, Matthew Chance, Paula Newton and Peter Wilkinson and journalist Livia Borghese contributed to this report.