Judge Alessandro Nencini also suggested in an interview with Corriere della Sera published Saturday that the decision of Knox's ex-boyfriend and co-defendant, Raffaele Sollecito, not to testify may have worked against him.
"It's the defendant's right, but it certainly deprived the process of a voice," Nencini was quoted as saying. "He limited himself to spontaneous declarations. He said only what he wanted to say without letting himself be cross-examined." Knox did not appear at the trial, but sent a letter to the court saying she feared wrongful conviction.
The newspaper said Nencini consented to the interview because he knew the sentence would create a media storm. The case has been top international news since Kercher was found in a pool of blood with her throat slit on Nov. 2, 2007, in the apartment Knox and Kercher shared in the university town of Perugia.
As the case has moved through Italy's court system, prosecutors have offered differing explanations for Kercher's killing, asserting in the first trial that Kercher was killed when an erotic game went awry and in the latest trial saying the violence was rooted in a longstanding disagreement over cleanliness. Both Sollecito and Knox deny involvement.
Nencino did not give a specific reasoning behind the verdict, saying the court settled on a motive that would be made clear in the written explanation, expected within three months.
Nenci, another judge and six lay jurors reinstated the guilty verdicts on Thursday against Knox and Sollecito that were first handed down in 2009, sentencing Knox to 28 1/2 years and Sollecito to 25 for the murder. An appeals court had acquitted the pair in 2011 and ordered them freed from prison, but Italy's supreme court threw out the acquittals and ordered a third trial, in Florence.
Lawyers for both Knox and Sollecito have said they would appeal, saying there was no proof that the two had committed the crime. Knox has said she will never willingly return to Italy to serve any sentence if the verdict is upheld.
Nencini said the court worked long and hard to process what he called a "half-room" worth of documentation in these months. Asked if the final verdict was unanimous after 12 hours of deliberations, Nencini hedged, saying it was a "shared" decision.
"I can say that in all these months, and in particular in the last meeting, we sensed the gravity of a sentence against young people and entire families," he was quoted as saying. "This is something that has affected many lives."
"I feel liberated because the moment of the decision is the most difficult," he was quoted as saying. "I also have children, and inflicting a sentence of 25 and 28 years on two young people is emotionally very tough."
A third person, Rudy Guede, was convicted in a separate trial and is serving a 16-year sentence.
Nencini hinted at what the court had found to be the most plausible explanation for what happened, saying that up until 8:15 p.m. on the night of the murder, Knox and Sollecito had other plans: In Knox's case, she was supposed to have gone to work at a bar, and Sollecito was supposed to have gone to a train station to pick up a friend's luggage.
"At the moment I can say that up until 8:15 of that evening, the kids had other plans, but they skipped them and an opportunity was created," Nencini was quoted as saying. "If Amanda had gone to work, probably we wouldn't be here."
While the changed plans that night have been well established by evidence presented to the courts, Nencini didn't explain how those details factored into a motive for the murder.
He justified not imposing any restrictive measures on Knox, who remains free in the United States. If necessary, the court would re-evaluate the measures imposed on Sollecito to prevent him from leaving the country, he said.
Sollecito had attended the morning session of Thursday's court hearing but then drove to Italy's northern frontier with Austria and Slovenia. While the judges and jury deliberated, he and his girlfriend visited Austria, but came back to Italy to spend the night.
In an interview with U.S. broadcaster NBC News on Friday, Sollecito said he wasn't trying to flee Italy by going to Austria. He said he had been planning to take a trip outside Italy if acquitted, but turned back as soon as he learned he had been convicted.
"I didn't want to flee, or to get away because I actually went back," he said.
He said he checked into the first hotel once back in Italy because he was tired. Police found him there Friday morning, and confiscated his passport and ID papers, as mandated by the court, but then set him free.