Hillary Clinton drawn into row over conviction of Amanda Knox

She said that Knox had been the victim of “harsh treatment”, a reference to claims – denied by Italian police – that she was put under pressure during 53 hours of interrogation after the murder and twice struck on the head.

Asked on US television about the trial Mrs Clinton said she had not had time to study the matter and could not offer an opinion, but would meet with Senator Cantwell “or anyone who has a concern”.

Headlines in today’s Italian newspapers included “Hillary steps in to the Amanda case” and “Clinton intervenes in Perugia trial”.

Groups set up in Seattle to lobby on behalf of Knox after her arrest two years ago urged Americans to petition President Obama to support her appeal and to boycott Italian holidays and products.

Giuliano Mignini, the chief prosecutor in Perugia, said he stood by his prosecution of Ms Knox, 22, who was given 26 years in prison, and Raffaele Sollecito, 25, her Italian former boyfriend, who was given a 25-year sentence.

“My conscience is clear,” Mr Mignini said. “This senator should not interfere in things she knows nothing about.”

He said that he resented accusations from the other side of the Atlantic that the investigation had been superficial and that no proof had been found to prove the defendants’ guilt. This was unacceptable, he said.

Mr Mignini said the evidence had been weighed not only by the judge and jury during the 11-month trial but also by investigators and the pre-trial judge. He said Ms Knox’s accounts of what happened had changed several times and had been “implausible from the very beginning”.

Mr Mignini said US critics claimed the jury should have been segregated for the entire trial to avoid being influenced by the media. “But how could we have segregated two judges and six jurors for 11 months?”

Claudia Matteini, the judge who sent Knox and Sollecito for trial last year, said she was surprised by the American criticism, which was “offensive”.

She added: “Don’t forget that because of a false accusation by Amanda Knox an innocent man spent 15 days in jail,” she said in a reference to Diya “Patrick” Lumumba, a Congolese bar owner whom Ms Knox accused of the murder in a partial confession which she later withdrew.

Beatrice Cristiani, the deputy judge at the trial, commented drily: “As far as I am aware our system of justice does not make provision for interference from overseas.”

In an editorial, Corriere della Sera said: “The first rule when an American citizen is accused of a crime abroad is that it does not matter whether he or she is innocent or guilty, what matters is that he or she has a US passport, which carries even more weight than an alibi.”

The paper recalled a series of incidents in which Americans had “escaped justice” in Italy. They included an episode at Cermis in the Italian Alps in 1998 when a low-flying US fighter pilot sliced through the cable of a ski lift, killing 20 people, and the death in 2007 of Nicola Calipari, an Italian secret agent who was shot at a US roadblock at Baghdad airport as he escorted a freed Italian hostage to safety.

It said that if Knox had been tried in America she would have faced the death penalty, which did not exist in Italy. The US in any case had no right to criticise Italian justice when “it has still failed to close down Guant?namo Bay”.

However, Lucia Annunziata, an Italian expert on US affairs, said: “In the absence of certainties, Italian trials often resort to the theory that the guilty party is not someone who indisputably committed an evil act but someone who could have done so or wished to do so.” This led to a reliance on psychological profiling, “which can be used or abused”.

La Stampa said that Mrs Clinton had had no option but to react to a wave of protest in the US, but would have to walk a careful line to avoid a diplomatic rupture with Rome. Luciano Ghirga, Knox’s lawyer, said he did not question the validity of the trial, which had been conducted correctly.

However, the defence would seek an acquittal on appeal on the grounds that no concrete proof or motive had been established. Mr Ghirga noted that while the jury had convicted Knox and Sollecito of murder and sexual assault, they had been acquitted of the charge that they stole €300 from Ms Kercher, which the prosecution had maintained was one of the points of friction between Knox and Ms Kercher – the American allegedly stole the missing cash to pay for drugs.

Mr Ghirga said this had removed the supposed economic motive for the crime. Rudy Guede, the Ivory Coast immigrant sentenced to 30 years last year for his part in the crime, had not been convicted of stealing the money either. “So who did steal it?” Mr Ghirga asked.

The first of two appeals allowed for under Italian law will be held in Perugia, probably next autumn. If it fails a second appeal will be made to the Court of Cassation (Supreme Court) in Rome.

Knox and Sollecito have been placed under 24-hour “suicide watch”.

Ms Kercher, 21, from Coulsdon, Surrey, was reading European Studies at Leeds University and had just begun a year in Italy as part of the Erasmus programme when she was killed in November 2007. She was found semi-naked in a pool of blood under a duvet in her bedroom, with her throat cut.

The prosecution claimed she had been tortured and killed for resisting a drug-fuelled sex game that escalated into violence. However, much of the DNA evidence linking them to the crime scene was challenged as unreliable.

Prosecutors painted a picture of the American student as “a manipulative and unscrupulous she-devil”, an image her family and the defence argued was character assassination at odds with the reality of a bright and normal “girl next door” American college student.