It was the third verdict in the case for Knox, 26, and Raffaele Sollecito, 29, who were found guilty in 2009 of killing Kercher. That conviction was overturned two years later on appeal, only to have the decision later overruled.
Knox was not in court Thursday when Judge Alessandro Nencini sentenced her to 28-1/2 years in prison, more than the 26 years she received at her first trial. She refused to attend the second appeal, which opened in Florence last year, writing to the court from Seattle that she feared being "wrongly convicted."
Sollecito was sentenced to 25 years. He was instructed to hand over his passport and forbidden from leaving the country before Italy’s supreme court confirms the sentence.
If the guilty verdict is upheld by the supreme court, Knox could face extradition proceedings. But under Italy’s slow-paced justice system, the supreme court is unlikely to hear the case until April or May 2015, said Francesco Maresca, a lawyer for the Kercher family.
Shortly before the jurors took the case Thursday, Knox's lawyers called on them to not overlook mistakes made by investigators. “We are anxious about your verdict,” lawyer Luciano Ghirga told the judge and jurors moments before they filed out to consider it.
Prosecutors had called for sentences of 26 and 30 years for Sollecito and Knox, the exchange student from Seattle who shared a house in the Italian town of Perugia with Kercher, then 21, who was found partially naked in a pool of blood with her throat slashed.
In his final summing-up, a second lawyer representing Knox, Carlo dalla Vedova, lambasted what he called "mistakes" made by forensic investigators. DNA found on a knife in Sollecito’s kitchen and attributed by prosecutors to Kercher could not be verified, Ghirga said.
Losing his customary cool, he said, "Italian sentences don’t depend on probability, but on certainty."
Knox and Sollecito spent four years in jail before being acquitted on appeal in 2011. However, Italy’s highest court overturned the acquittal and ordered a new appeal, saying the first was riddled with “shortcomings, contradictions and inconsistencies.”
“Amanda has been in touch with us daily,” said Dalla Vedova. “She is very nervous and worried since her life is again jeopardized."
In an interview with Italian television Wednesday, Knox said she would be waiting at home with her family for the verdict with "my heart in my mouth."
"The proof is in the facts. There is no proof I was there when it happened," she said.
Patrick Lumumba, a Congolese barman who was briefly jailed when Knox accused him of the murder, took another view of her absence.
"She is guilty, and she is running away from Italian justice," he said on the sidelines at the Thursday hearing.
The Italian newspaper Corriere della Sera reported that Kercher's sister, Stephanie, had refused to read a letter sent to her this week by Knox.
Sollecito vacationed in the Dominican Republic during the course of the trial, prompting suspicions that he planned to flee Italy, because the Caribbean island has no extradition treaty with Rome. But on Thursday morning, he was in court.
“He wanted to stay home in southern Italy, but we said, 'Everyone thinks you are going to flee,' and he said 'OK, I'm coming,'" said Giulia Bongiorno, a lawyer representing him.
"He has decided to face the situation. He is afraid of nothing, and his presence is a silent scream of innocence," said Luca Maori, another attorney for Sollecito.
Kington is a special correspondent.