Knox stayed in the United States as lawyers geared up for the second retrial in Florence, Italy, for the 2007 death of Knox's British roommate Meredith Kercher, CNN reported. Her Italian boyfriend at the time, Raffaele Sollecito, was also absent at the onset of proceedings. His father expressed optimism the appeals court decision tossing out the twin convictions would be upheld after Italy's Supreme Court ordered the matter to be taken up again by the court that freed the couple in 2011, the ANSA news agency said.
Knox and Sollecito have maintained their innocence and Knox has said she'd be willing to take a polygraph. Sollecito declined to enter a deal with prosecutors implicating Knox in Kercher's death before the start of the most recent retrial, saying he knows she's innocent.
"I did it because I know it's the truth," he said of turning down a plea deal. "It's the good thing to do. It's the only way for me."
The two were convicted in 2009 of killing Kercher, a 21-year old British exchange student found stabbed to death in November 2007 in the house the two women were renting while attending classes in the Italian university town of Perugia.
The convictions of Knox and Sollecito were overturned in 2011 and Knox returned to her hometown of Seattle after being released from an Italian prison.
The Italian Supreme Court decided last year to return the case to the appeals court for retrial, saying the jury that acquitted Knox didn't consider all the evidence, CNN said.
The court's decision to retry the case "may be interpreted by the American authorities as double jeopardy -- twice tried for the same fact or the same case," Riccardo Montana, a law lecturer at City University in London, told CNN. "In Italy it's not like this, because this is still the same trial."
That's precisely what ABC News reported Knox's lawyer, Carlo Dalla Vedova, asked at the outset of Monday's retrial: "[Is Knox] being treated like other defendants? ... Is this constitutional that she be tried endlessly?"
Sollecito, in an interview with NBC, echoed those sentiments.
"It seems to be a pretty never-ending saga of a nightmare," Sollecito said. "My life is still on hold and I cannot move on. I cannot make plans for my future. I don't see any real future for me instead of standing in the trial, kind of forever."
If she is convicted, Knox will be ordered to return to Italy. If she refuses, Italy could file an extradition request with the United States, CNN said, but it was unclear if U.S. officials would comply.