(CNN) -- A probe into the evidence against Amanda Knox, the American student convicted of murder in Italy, was extended for another 40 days on Saturday, according to her attorney.
The results of new DNA were due Saturday, but forensic experts were not able to complete their tests in time.
Knox, 23, was sentenced in December to 26 years in prison for the death of Meredith Kercher at the villa the two shared in Perugia, the central Italian town where both were students.
Kercher, 21, was found dead in November 2007, semi-naked with her throat slashed. Knox and her former Italian boyfriend, Raffaele Sollecito, were both found guilty of the murder.
Sollecito is serving a 25-year sentence. A third person, Rudy Guede, a drifter originally from the Ivory Coast, was tried separately and is serving a 16-year prison sentence.
Speaking in Italian, Knox made another emotional plea to the court Saturday, said her attorney Luciano Ghirga. Knox insisted to the jury that she is innocent and has already spent three and a half years behind bars.
She also said she is grateful for the court's pursuit of the truth, Ghirga said.
Knox's father, Kurt, was in court and later told CNN that, "I know Amanda is upset about continuing to stay in prison but is thankful to the court for the full evaluation."
The forensic experts retesting evidence as part of Knox's appeal are now scheduled to present their results on June 30 to a jury composed of two judges and six citizens.
That evidence includes a knife found in Sollecito's apartment with Knox's DNA on the handle and what Perugia prosecutors say is Kercher's DNA in a tiny groove on the blade.
The prosecution contends that the knife was used to stab Kercher in the neck and that it had been cleaned. The DNA matter attributed to Kercher consists of flesh, not blood, they say.
The sample, however, was so small that forensic scientists investigating Kercher's murder were not able to double test it in accordance with international forensic science norms, which Knox's legal team says raises doubts about its validity.
The second piece of evidence the forensic experts will test is the tiny metal clasp from Kercher's bra, which was cut from her body after her slaying. Forensic scientists in the investigatory phase determined that Sollecito's DNA is present on the metal clasp.
The clasp was identified on an investigatory video on November 2, 2007, when Kercher's body was found. But the clasp was not collected until nearly six weeks later, giving the defense cause to question whether the sample may have been contaminated.
Prosecutors in Perugia said Knox directed Sollecito and another man infatuated with her, Guede, to hold Kercher down as Knox played with a knife before slashing Kercher's throat.