"The only explanation I can give myself: If it turns out he really is involved in a direct way in this tragedy, it means he was terrorized and so he reacted in this way," Finnegan Lee Elder's mother Leah told Italian newspaper La Stampa.
Meanwhile Elder's father Ethan has visited his son for the first time since the crime. Finn Elder, 19, and Gabriel Christian Natale-Hjorth, 18, both of San Francisco, are accused in the stabbing death of Italian policeman Mario Cerciello Rega in Rome last week.
Cerciello Rega had recently returned from his honeymoon and not been scheduled to work when he and the partner were sent out in plainclothes to work a case involving an alleged extortion attempt over a drug deal, police have said.
The teens have each confessed to their respective involvement in the crime, authorities say. Elder is accused of stabbing Cerciello Rega 11 times, and Natale-Hjorth is accused of punching Cerciello Rega's partner. Under Italian law, Natale-Hjorth could also be tried for murder for participating.
RELATED: American students confess to fatally stabbing Italian policeman Mario Cerciello Rega: Investigators
Meanwhile, a new Instagram photo has emerged on Italian media of Elder, back in San Francisco before his trip, which shows the teen holding a large knife. Italian media report that that photo, along with one of Elder in front of the Pantheon, are now part of the investigation.
SEE ALSO: Finnegan Lee Elder: Who is the American student accused of fatally stabbing Italian officer Mario Cerciello Rega?
Natale-Hjorth's father, Fabrizio Natale, says his son did "never imagined there would be a confrontation and did not know his friend was armed." Natale also visited his son in custody.
"We are very upset by the predicament he's in, but we are all fully convinced of his innocence," Natale said in the statement issued through his lawyer.
Natale described his son as a "normal" teenager who was studying to become an architect.
"He is devastated by the Carabinieri's death, and I, as a father, cannot help but feel the grief of (Cerciello Rega's) family," Natale said.
The Associated Press and ABC News contributed to this story.
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