The Italian court that found American student Amanda Knox guilty of murder in January, said late yesterday she had killed her British flatmate because of a domestic argument, rather than during a sex game, and that she herself had wielded the knife.

Knox spent four years in an Italian jail after a court found that she and her then boyfriend, Raffaele Sollecito, had murdered 21-year-old British student Meredith Kercher in 2007.

That conviction was overturned on appeal and Knox returned to the United States in 2011, but both were found guilty again at a retrial of the appeal.

Knox and Sollecito both proclaim their innocence. They are appealing again, and Knox has said she will not return willingly to Italy to serve the rest of her more than 28-year sentence.

A third person, Ivory Coast-born Rudy Guede, who was tried separately, is serving a 16-year sentence for his part in Kercher's murder at the university town of Perugia.

In its lengthy explanation of the January verdict, the Florence court said the theory that was initially used to convict the pair - that they had killed Kercher in a sex game gone wrong - was not plausible.

It was not "credible that the four young people began a group sex activity that Meredith Kercher later suddenly no longer wanted to pursue further," the court said in the "motivations" to its ruling, a document commonly issued by an Italian court many weeks after its verdict.

"This hypothesis is not compatible with the personality of the English girl," it said.

Instead, the murder occurred because Kercher and Knox did not have a good relationship and had an argument about flatmate issues on an evening when Knox and Sollecito had taken drugs, which then escalated, it said.

Knox, Sollecito and Guede attacked Kercher together and pushed her into her bedroom, the court said. Guede assaulted her to "satisfy his sexual instinct" while Sollecito and Knox participated "in a desire to abuse and humiliate the English girl."

The three then killed Kercher so that she could not report the sexual assault, the court said.

A knife discovered in Sollecito's house "was one of the two weapons used in the murder, and was the one that was held by Amanda Marie Knox, who wounded Meredith Kercher on the left part of her neck, causing the only fatal wound," it said.

Knox in a statement maintained her innocence and took issue with the findings of the court in its document outlining what it holds to be the motivations in the crime.

"The recent motivation document does not - and cannot - change the fact that the forensic evidence still does not support my participation and the circumstantial evidence still remains unreliable and contrary to the conclusion of guilt," Knox said.

Knox and Sollecito - who is free on bail but is not allowed to leave Italy - are fighting their convictions in a final appeal process expected to conclude in 2015.

"I remain hopeful that the Italian courts will once again recognize my innocence," Knox said.