An Italian state police forensic expert who carried out the initial DNA exam presented at Amanda Knox murder trail insisted there was no contamination on crucial pieces evidences linking the American student and her co-defendant with the murder of her British roommate.

Patrizia Stefanoni examined DNA traces on items collected from the crime scene in the aftermath of the 2007 killing of Meredith Kercher, who was studying abroad in the Italian city of Perugia. While prosecutors used her testimony to help convict Knox and ex-boyfriend, Raffaele Sollecito, in 2009, her work has been criticized by court-appointed experts who claim there were glaring errors in the evidence gathering process -- including on a knife considered to be the murder weapon -- that suggests it may have been contaminated.

On Tuesday, Stefanoni asserted that DNA found on the knife definitely contained Kercher's genetic profile and insisted the weapon was not polluted during the collection process.

An independent review of DNA traces in the first trial found that much of the evidence collected in the original investigation fell below international standards, which could have tainted some of the samples. The review concluded that due to the risk of contamination and the low amounts of DNA used for testing, it was impossible to extract a genetic profile with any certainty.

In the first trial, prosecutors argued that Knox's DNA was found on the knife's handle and Kercher's DNA was found on the blade. They also maintained a mixture of Kercher and Sollecito's DNA was detected on the clasp of Kercher's bra.

However, professors Carla Vecchiotti and Stefano Conti, both court appointed forensic experts, said the clasp from Kercher's bloodied bra was dirty and contained such a mix of DNA that a high number of genetic profiles could potentially be extracted, depending on how one combined the data.

Moreover, the pair argued the clasp had been allowed to rust, making further testing pointless.

I could find yours, too, Vecchiotti told the presiding judge, according to MSNBC.  I'm there, too, she said, adding that some data was compatible with her own DNA.

Vechiotti said Kercher's DNA profile was the only one undoubtedly located on the clasp.

The findings have given the defendants their first real chance at gaining their freedom after spending almost four years in prison.

I think we're feeling more hopeful, Knox's father Curt Knox told NBC on Tuesday.

Knox and Sollecito were convicted of sexually assaulting and murdering Kercher in the apartment she and Knox shared while studying in Perugia. Both defendants deny any wrongdoing and have appealed the verdict. A third person, Rudy Hermann Guede, was also convicted in Kercher's murder after it emerged that his fingerprints were found at the crime scene.

Prosecutors alleged that Knox and Sollecito stabbed Kercher in the neck after she refused to take part in an orgy on the night of Nov. 1 2007. Both have denied the charges and claim they do not know what caused Kercher's death.

Stephanie Kercher, Meredith's sister, reportedly wrote an impassioned letter to the Italian judges hearing Knox's and Sollecito's appeal.  According to The Guardian, the letter aligns with the family's view that the co-defendant's were responsible for Kercher's death, stating that, it is extremely difficult to understand how evidence gathered with care and presented as valid at the original trial now risks becoming irrelevant.