"She's absolutely innocent," Mellas said. "There's no doubt in my mind, never has been, that she's had nothing to do with this."
Knox, an honor student from Seattle, was studying in Perugia in November 2007, when her 21-year-old British roommate, Meredith Kercher was discovered murdered in the house they shared.
"There were 47 separate wounds - bruises, scratches, cuts," said British journalist Nick Prisa. "The police and prosecution has always insisted that this was a sex game gone horribly wrong."
Police became suspicious of Knox and her boyfriend, Raffaele Sollecito. During questioning, investigators say Amanda signed a statement admitting she was in the house when Meredith was killed. She also falsely accused a local bar owner as being the killer.
But why would Amanda lie to the police? Well, it turns out that her so-called confession came after an all-night 14-hour interrogation where she was denied food, water, sleep and legal representation. And according to recorded testimony in court, Amanda says she was hit during the questioning.
"I was hit in the back of the head by one of the police officers who said she was trying to make me help me remember the truth," Amanda said in court. "I was very, very scared because they were treating me so badly and I didn't understand why."
"They psychologically terrorized her until she signed it," said private investigator Paul Ciolino, who spent weeks in Italy looking into the case for 48 Hours Mystery.
He says all of the prosecution's forensic evidence is inconclusive.
"They don't have a case," Ciolino said. "There's no physical evidence. There's no eyewitness testimony. There's no motive."
In fact, Ciolino says the real killer is Rudy Guede, whose DNA and bloody fingerprints were found in Meredith's room. Police believe Amanda and Raffaele joined Guede in the sex orgy that led to Meredith's death. Guede was tried separately earlier this year and found guilty of Meredith's murder.
"Rudy Guede raped and murdered Meredith Kercher," Ciolino said. "Amanda and Raffaele never laid eyes on Rudy, never met with him and never hung out with him... didn't know him."
The trial is expected to last into the fall. If convicted, 21-year-old Amanda Knox could face life in prison.