My daughter, Amanda Knox

The stepfather of the American student on trial for Meredith Kercher's murder accuses the Italian police of beating her during questioning

Amanda Knox, the American student on trial in Italy for the murder of her British flatmate Meredith Kercher, was repeatedly beaten during the all-night interrogations that led to her being named as a suspect in the brutal killing, her stepfather has claimed.

In an extended interview with The Independent on Sunday, Chris Mellas, 36, an IT professional, said that Ms Knox's confession that she had been in the flat when Ms Kercher died had been forced out of her. "It was coercion," he said. "They did what they needed to do to get her to say what they wanted her to say." He claimed that one of the policewomen, who allegedly hit her on the head, faces six charges of beating other suspects during interrogation in earlier cases.

Mr Mellas also alleges that the interpreters made available to his stepdaughter were not neutral, but police officers with a little English, who participated actively in the interrogations. The police, he says, admitted that they had no intention of letting her see a lawyer during questioning. "In Italy you have a right to a lawyer as you do in the United States," he said. "But they told her that if she had one it would make it bad for her. Even if she had insisted, they would not have allowed it. One of the police witnesses admitted as much on the stand."

Ms Knox's family and others, including a lawyer, have formed a support group which maintains a constant presence at the twice-weekly hearings and keeps the public at home aware of her situation. Mr Mellas is nearing the end of his latest stint in Perugia, sitting a few rows behind his stepdaughter in court. "The stress on my family is excessive," he said. As we spoke, his phone rang constantly with calls from American TV networks and other journalists. "The trial dominates your life completely. You wake up and it's the first thing on your brain, and it's the last thing when you go to sleep, and frequently it also bothers you while you're sleeping."

Ms Knox and her co-accused, her Italian ex-boyfriend Raffaele Sollecito, have been in jail for more than 16 months now, and their trial is still in the early stages. And although neither has yet been found guilty of anything, Mr Mellas maintains they are treated like criminals. "I look at the overall process and I wonder, where is the presumption of innocence?" he said. "She's in jail because they are afraid of her leaving the country. Well OK, so detain her. But on top of this they apply all the rules of the convicted to her. So when I go in I can't take her a pair of socks. Why not?"

The third suspect in the case, Rudy Guede, a Perugia resident originally from the Ivory Coast, was jailed for 30 years at the end of a "fast-track" trial before Christmas, after the prosecution produced an overwhelming amount of forensic evidence of his presence at the crime scene.

The prosecution maintains that Guede was just one player in the sex orgy that they say culminated in Ms Kercher's death. Yet two months into the new trial, no evidence comparable to that which damned Guede has emerged: no forensic trace at all of Ms Knox at the scene, and of Mr Sollecito only traces of his DNA, hotly disputed, on the clasp of Meredith's bra, found six weeks after the crime.

If all three accused were involved, why such abundant evidence of Guede at the scene and such paltry evidence of Ms Knox and Mr Sollecito? Chris Mellas says the explanation is simple: his stepdaughter had nothing whatever to do with the crime. She spent the night with her boyfriend at his flat.

He says that the reason another version emerged – in Ms Knox's confession, that she had been in the kitchen of her flat when Ms Kercher died, closing her ears to her flatmate's screams – was because she was bullied and beaten until she said what the police wanted to hear. "They screwed with Amanda's head so bad that night she didn't know what was right and wrong," he said. "In a statement she wrote the next day she says, 'I don't know much any more, I'm so confused. But I know that I did not kill Meredith.'"

In the absence of hard evidence, witness after witness has testified to the strangeness of Ms Knox's behaviour after the murder: her apparent lack of emotion, how she did yoga exercises and even turned cartwheels in the police station. How does Mr Mellas account for all that?

"She was trying to be this strong person," he replied, "trying to pick herself up and help the police and get on with life. Unfortunately it didn't work out because it took a toll on her: you suppress your grief and shock long enough and you kind of crumble."

As the marathon trial meanders on, Ms Knox's behaviour has intrigued onlookers. On Valentine's Day she came into court wearing a T-shirt with "All you need is love" in huge letters. In her first "spontaneous declaration" before the court, she explained that the vibrator in her bathroom, spoken of in shocked tones by English friends of Ms Kercher, was a gift. Mr Mellas called the T-shirt "the Valentine's Day debacle", adding: "In reality did it hurt anybody? No. But there is a certain level of decorum for certain situations which is required to be maintained. And she's completely missing the boat on that one. But that's Amanda!" And the vibrator? "It's two inches long," he said. "It's a little toy on a keychain. One of her friends bought it as a gag."

What makes Amanda Knox tick? "I'll give you an example," her stepfather said. "When she was about 14, we were at a soccer tournament and there was this enormous puddle of chocolate-brown muddy water. And her coach looked at Amanda and said, 'I bet you wouldn't jump into that mud puddle. I'd give you five bucks if you did.' And he hadn't even finished the sentence and she went and bellyflopped into it and splashed everybody. These are the things that are just her; she has a unique quality to her."

The essence of his case is that Amanda Knox may be extremely scatty, woefully insensitive even, but no murderer. And so far no one has proved otherwise.