A 19-year-old US tourist has confessed to the murder of an Italian policeman, Italian media said on Saturday.
The tourist was travelling with an 18-year-old friend, who has also been arrested for alleged involvement in the killing.
Mario Cerciello Rega, 35, was stabbed to death in central Rome in the early hours of Friday morning, just weeks after returning from his honeymoon.
He was called to the scene after reports of a robbery.
Italian media initially said the murder suspects were North African.
The young men were allegedly in the Trastevere area trying to buy drugs.
They are said to have stolen a rucksack from a drug dealer who had sold them fake product, according to Italian news agency Ansa.
They reportedly offered to bring it back to him, if he paid them $100 (£124; €111).
As they waited, they were approached by Rega and a colleague as part of a plain-clothed operation because the police had been tipped off about the bag exchange, Ansa reported.
A brawl ensued, during which Rega was stabbed multiple times. He was taken to hospital but died of his injuries.
The two Americans were picked up at a hotel by police on Friday morning.
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Vice-Brigadier Rega had been married only 43 days and had returned from his honeymoon just this week.
“Mario was a lovely lad,” Sandro Ottaviani, commander of Rome’s Piazza Farnese Carabinieri station, was quoted as saying by Ansa.
“He never held back at work and he was a figurehead for the whole district. He always helped everyone. He did voluntary work, accompanying sick people to Lourdes and Loreto. Every Tuesday he went to Termini train station to feed the needy.”
His funeral will be held on Monday, in the same church in which he was married.
The killing shocked Italy and prompted tributes from across the country.
On social media pages, the Carabinieri paid tribute to Mario Cerciello Rega’s “unconditional and brave dedication”, and said his loss would be felt by his 110,000 fellow Carabinieri officers.