Mother's 'nightmare' after US teen admits killing Italian cop

The mother of a US teen detained in Rome over the killing of an Italian police officer spoke Wednesday of her shock at the arrest of her "thoughtful boy", as the suspect's father arrived in Italy.

Finnegan Elder, 19, has been charged with aggravated homicide along with friend Gabriel Natale Hjorth, 18, following the death of officer Mario Cerciello Rega, who suffered multiple knife wounds in Friday's attack.

Elder has confessed to stabbing Cerciello, 35, with a US Marine partially-serrated, close-quarters combat knife, according to police. But says he mistook the plain-clothed officer for a dangerous drug dealer and used the weapon in self-defence.

His father Ethan Elder said on arrival in Rome that "the first thing I need to know is how to get into prison to see my son," before heading directly with his lawyer to the city's Regina Coeli jail, according to Italian media reports.

"We feel like our world has come crashing down," Elder's mother Leah said in an interview with Italy's La Stampa daily.

"I don't know how to describe it. It's like a nightmare we'll wake up from".

Their son was high on a mix of spirits, beer and prescription medicines when the attack took place, police said Tuesday. Cerciello had tackled him to the ground during a nighttime drugs raid in a genteel Rome neighbourhood.

"Finn is a thoughtful boy. The only explanation I can give, if he really is involved directly in this tragedy, is that he was terrified and therefore reacted rashly," the suspect's mother said.

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