Amanda Knox has declared she would never go back to Italy willingly and would fight her reinstated conviction for the murder of Meredith Kercher "until the very end", as an emotional public battle over her possible extradition began in earnest a day after the guilty verdict was returned.

An emotional Knox vowed that she would fight tooth and nail to clear her name. "I'm going to fight this until the very end. It's not right, and it's not fair and I'm going to do everything I can," she said, through tears, in an interview with ABC's Good Morning America.

Asked if she was prepared for extradition, should it happen, she said: "I'm not … This really has hit me like a train. I did not expect this to happen.

"I really expected so much better from the Italian justice system. They found me innocent before. How can they say that it's guilt beyond a reasonable doubt?"

Meanwhile, Kercher's relatives said they would fully expect an extradition to take place if the sentence became definitive next year.

"If somebody – whoever that may be – can be convicted in a court of law and found guilty, that person should be punished as is appropriate," Lyle Kercher, Meredith's brother, said.

Refusing an extradition could cause the US problems "going the other way [with Italy] and probably with other countries", he said.

"I'm sure the American government try to extradite a lot of convicted criminals from abroad themselves so I guess they'll set a precedent if they didn't uphold their own laws."

However, many legal observers think it is highly unlikely that the US would let the extradition of Knox go ahead.

Knox, 26, and her former boyfriend, Raffaele Sollecito, were cleared on appeal in 2011 of killing Kercher, 21, a British exchange student in Perugia. But Italy's top appeal court, the court of cassation, quashed that ruling last year.

On Thursday night, after nearly 12 hours of deliberations, Knox was handed a 28-and-a-half-year sentence, while Sollecito, 29, was given a 25-year sentence. Both sentences were suspended pending appeal.

Sollecito, meanwhile, was found by police in a hotel in north-east Italy near the border with Austria, sparking speculation about his intentions. "How do I feel? I would like others to put themselves in my place," he told the Italian news agency Ansa.

In its verdict, the Florence appeal court had ordered Sollecito to stay in the country and have his travel documents confiscated. Sollecito denied he had been planning to escape Italy, saying he had gone to Austria for a "trip" and had then returned when he heard the court's ruling.

A senior police officer said Sollecito, who disappeared from the courtroom on Thursday before the verdict was read, had spent the night with his girlfriend at a hotel in the village of Venzone, 40 miles from the border crossing. They had arrived together at 1am, he said.

"He has been cautioned that he is forbidden to leave the country," the police spokesman said. "His passport has been taken away from him and his identity card has been stamped to show that he must not leave Italy."

"As a free man, I was able to move around as I liked," Sollecito was quoted as saying. "Then I heard the sentence and I came back to Italy straight away. I was tired and I stopped at the first possible place."

One of his defence lawyers, Luca Maori, said his client had told him he had "never thought of fleeing. Not before and especially not now".

The next stage in the long-running case is expected in the spring of next year, when the court of cassation is due to examine the appeals being brought by Knox and Sollecito.

It could either uphold the verdict and make the convictions definitive – in which case Italy could request Knox's extradition and Sollecito would be facing a return to jail – or send the case back for another appeal.

Speaking in Florence, Stephanie Kercher, Meredith's sister, said Thursday's ruling had just been a "step" on the path to a definitive conviction. But her family were resigned to the fact they might always remain in the dark about what happened on the night of 1 November 2007, she said. "I think we're still on a journey to the truth," she said. "It may be we don't ever really know what happened that night and that's something we'll have to come to terms with."

Rudy Guede, from Ivory Coast, has been definitively convicted of the murder and is serving a 16-year sentence in an Italian jail, but judges said they did not think he acted alone.

In her interview, Knox said she had sent the Kerchers a letter. "Mainly I just want them to know that I really understand that this is incredibly difficult," she said. "That they have also been on this never-ending thing and that when the case has been messed up so much, like a verdict is no longer consolation for them. And that just the very fact that they don't know what happened is horrible."

Stephanie Kercher said that while she had been told of the letter's existence she did not think she would be reading it. She also said she was unlikely to ever want to meet Knox, regardless of the legal outcome.

But Lyle acknowledged that the Seattle-based student was right when she said the verdict brought them little consolation. "No matter what the decision and whether it is finally upheld or not, nothing of course will ever bring Meredith back. Nothing will ever take away the horror of what happened to her," he said. "The best we can hope for is of course finally bringing this whole case to a conclusion … and then everyone can move on with their lives."