Rome (CNN) Two Americans charged with aggravated murder and extortion in the stabbing death of an Italian police officer last July went on trial Wednesday in Rome, claiming self-defense.

Finnegan Elder, now 20, and Gabriel Natale Hjorth, now 19, say Italian paramilitary officer Mario Cerciello Rega and his partner threw the first punches in a 32-second scuffle that ended in Rega's death July 26. Both officers were working in plainclothes.

The presiding judge, Maria Sabina Calabretta, heard from scores of legal representatives in the hourslong trial to determine what evidence would be introduced. In Italy, criminal trials allow civil parties to participate, meaning they can offer evidence and cross-examine suspects and other witnesses.

Among those Calabretta allowed as civil plaintiffs for the upcoming trial is the widow of the officer, Rosa Maria Rega, who will be allowed to defend her dead husband's honor, which has been called into question over the handling of the attempted arrest of the Americans.

Calabretta also allowed Italy's defense and interior ministries to participate because Rega, 35, was part of the paramilitary police force.

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