Finnegan Elder, 19, and Gabriel Natale Hjorth, 18, have been charged with aggravated homicide and attempted extortion following the killing of officer Mario Cerciello Rega, who suffered multiple knife wounds in Friday's attack.
Elder has confessed to stabbing plainclothes officer Cerciello, 35, with a US Marine partially-serrated, close-quarters combat knife,
police said, as they gave a detailed account of what happened on the night of the killing.
Natale held Cerciello's partner Andrea Varriale down during the attack in Rome's upmarket Prati neighbourhood, they said.