Knox was first convicted, along with her ex-boyfriend Raffaele Sollecito, of killing her British roommate Meredith Kercher in 2007.
Both were then acquitted after a second trial in 2011.
Knox is now back in Washington state, and called Tuesday's ruling "painful," but said she's confident she'll be exonerated.
Amanda Knox returns to US after 4 years in prison
Scott Pelley: Why did the Italian court order a new trial?
Peter Van Sant: It all comes down to evidence and more specifically evidence that was excluded from the second trial. The prosecution filed an extensive document saying all the DNA evidence should have been allowed into this trial instead it all focused on a bra clasp and the alleged murder weapon, a knife. Now they're going to get a third trial where much more DNA evidence will come in.
Pelley: Why was the DNA not allowed in the trial that acquitted her?
Van Sant: By American or British standards it simply did not rise to the level of professionalism. If you watch, there's video of them gathering this evidence. They pick up the bra clasp from the floor, they move it from hand to hand. Hand it to a colleague, put it back down on the floor where people had been in and out of that room for up to six weeks and so it's simply not considered evidence in the United States because it's contaminated.
Pelley: What does the bra clasp have to do with the case?
Van Sant: It was ripped from Meredith Kercher during this sexual assault and her murder, her throat was slashed and it was believed that there was DNA on that bra clasp that said Amanda Knox's boyfriend Raffaele Sollecito had ripped it off.
Pelley: Even though Italy wants to retry Amanda Knox it doesn't seem very likely that she's going to travel back there and put herself in jeopardy.
Van Sant: No, she has already said she will not going back. This will be a trial in absentia and this process potentially could take up to four more years of her life before it's resolved.