The third murder trial of Amanda Knox and Raffaele Sollecito got underway Monday in Florence, Italy, with new prosecutors and judges. But the stars of the show weren t there.

Knox was attending classes at the University of Washington in Seattle, where she s still trying to finish an education interrupted by four years behind bars in Italy.

And Knox s ex-boyfriend, Raffaele Sollecito, was in the Dominican Republic. Sollecito told NBC s Today Show he plans to attend the trial later.

If they ask me to testify, I will do that. It's the decision of the courts for me to testify or not, Sollecito said.

Previous trials focused on DNA evidence, and in Florence Monday the judge hearing the retrial ordered new testing of the kitchen knife allegedly used to murder Meredith Kercher.

Kercher was found with her throat slashed in November 2007 in the house she shared with Knox and two Italian women. Five days later, police arrested Knox and Sollecito for the crime.

It s expected the judge will review Knox s confessions, in which she admitted being in the house when Kercher was murdered. Knox claims police and prosecutors bullied her into making those statements, which she later retracted.

It s been a head spinning trip through the Italian justice system for Knox.

She was held in jail for almost a year while prosecutors built their case. She was indicted on murder and sexual assault charges in October 2008. Her trial opened in January 2009 and lasted nearly a year. In December 2009 Knox was found guilty of murder and sexual assault and sentenced to 26 years in prison. She appealed and in November 2010 her second trial began. That trial lasted another year and in October 2011 an appeals court exonerated Knox. Within hours of that decision she flew home to Seattle.

But it wasn t over. Prosecutors appealed the acquittal.

The prosecution in many other systems in the world has the ability to appeal acquittal, explained Associate Professor Mary Fan who teaches International Law at the U.W.

Prosecutors got what they wanted. In March, Italy's highest criminal court, threw out Knox s acquittal and ordered a second appeals trial.

It wouldn t happen in America where a person can t be re-tried on the same charge once they ve been acquitted.

It looks like double jeopardy in the U.S. context certainly, but our ideas of double jeopardy are not the same as in the international system and certainly not in Italy, Fan said.

So what if Knox is re-convicted and Italy moves to extradite?

A smart lawyer always can come up with arguments, and I can see arguments in her case, to try to defeat extradition, but it will be an uphill battle....for her to defeat extradition if there s a conviction, Fan said.

And it's unclear when the Italian courts will be finished with Knox. Even the outcome of this trial can be appealed.

Fan said that should Knox be re-convicted, Italy could request extradition under its treaty with the U.S. But, it s not a simple process. The request goes first to the U.S. State Department, then to a federal judge and finally back to state for a decision.

Prosecutors accused Knox and Sollecito of murdering Kercher in a drug fueled sex game involving a third man, Rudy Guede. In 2008, Guede was convicted and sentenced to 30 years in prison but his sentence was reduced to sixteen years on appeal. It s not known whether Guede will testify in the second appeals trial.

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