San Francisco teens Finnegan Lee Elder, 19, and Gabriel Christian Natale-Hjorth, 18, are suspected of fatally stabbing 35-year-old Vice Brigadier Mario Cerciello Rega 11 times last Friday.
At a Tuesday press conference, Italian police displayed the weapon the teens allegedly used: a 7-inch Ka-Bar developed for U.S. troops during World War II.
Colonel Lorenzo D'Aloia told reporters that Cerciello Rega was unarmed the night he was murdered but wouldn't have had time to draw his weapon before he was mortally wounded.
Gen. Francesco Gargaro of Italy's paramilitary Carabinieri police said Rega and a partner, Andrew Varriale were assigned Friday to respond to an extortion attempt involving a failed drug deal. Thieves had demanded money and cocaine in exchange for returning a stolen backpack, Gargaro said.
The officers were in plainclothes and identified themselves as Carabinieri as they approached the two suspects, but were immediately attacked, Gargaro said.
Elder and Natale-Hjorth were detained in the officer's slaying. Police have said Elder is suspected of stabbing Cerciello Rega and Natale-Hjorth is suspected of assaulting the other officer.
Varriale did have his gun, but after Natale-Hjorth stopped punching and scratching and ran off, the officer turned his attention to his wounded partner, Gargaro said.
According to the judge's written ruling, Elder and Natale-Hjorth allegedly paid for cocaine from a drug dealer in Rome's Trastevere neighborhood but the deal wasn't completed because police officers had approached.
Investigators said the two then snatched and ran off with the knapsack of the Italian man who had put them in contact with the dealer.
Police said when the intermediary called his own cellphone, which was in the stolen backpack, the teens told Sergio Brugiatelli they'd return the bag in exchange for $112 and a gram of cocaine.
After Brugiatelli reported the demand to police, an appointment with the teens was set up and Cerciello Rega and Varriale were sent to the rendezvous point.
Varriale said they identified themselves as Carabinieri and showed their badges, but were immediately attacked, Judge Chiara Gallo wrote in her ruling upholding the detention. The teen suspects told investigators they did not know the two men who showed up to meet them were police officers, Gallo wrote.
Elder told authorities during his interrogation that he stabbed Cerciello Rega because he feared he was being strangled, the judge wrote, noting the teen didn't have any marks on his neck.
Officers tracked the Americans to their hotel room and reported finding the alleged weapon, a 7-inch military-style attack knife, hidden in the room's drop ceiling.
Judge Gallo cited contradictions in the teens' statements to investigators: Elder told investigators that Natale-Hjorth hid the knife, while Natale-Hjorth said he didn't even know about a stabbing until his friend woke him hours later and reported he had "used a knife" and then washed it.
The Associated Press contributed to this report.