A police officer killed in central Rome last week was unarmed and unable to defend himself due to the speed with which he was attacked, Italian investigators have told reporters.

Mario Cerciello Rega died after being stabbed 11 times in the early hours of Friday morning. Two American teenagers, Finnegan Lee Elder and Gabriel Christian Natale Hjorth, are being held on suspicion of murdering the 35-year-old and injuring his colleague Andrea Varriale.

Mario Cerciello Rega. Photograph: AP

“He only had handcuffs with him,” the police commander, Francesco Gargaro, said at a press conference in Rome on Tuesday. “Even if he had a gun, there was no chance to use a weapon, the two officers were attacked immediately.”

Gargaro said Cerciello Rega, who was on duty in plain clothes, left his pistol at his barracks. “He wouldn’t have imagined that [the assailants] would be armed,” Gargaro said.

Elder, 19, is accused of killing Cerciello Rega, while Hjorth, 18, allegedly “pummelled” the second officer.

Police said Elder confessed to the killing after the alleged murder weapon was found in a hotel room the teenagers from San Francisco were sharing. Elder told police he had brought the 18cm (7in) knife in a suitcase from the US, but did not explain why.

Michele Pristipino with fellow prosecutor Nunzia D’Elia (left) and police commander Francesco Gargaro (right) at a press conference in Rome on Tuesday. Photograph: Andrew Medichini/AP

The prosecutor Michele Prestipino said that the teenagers’ rights were respected during questioning after a photograph emerged on Sunday of Hjorth blindfolded and handcuffed shortly after his arrest.

“The interrogations were carried out with respect for the law … with all guarantees of the defence, in the presence of defence lawyers and interpreters,” he said.

The funeral of murdered police officer Mario Cerciello Rega. Photograph: Antonio DiLaurenzio/Rex/Shutterstock

Hundreds of people, including Italy’s two deputy prime ministers, attended Cerciello Rega’s funeral on Monday, held in the same church where he was married less than two months ago. He had only recently returned to work after his honeymoon.

In a case that has shocked Italy, Cerciello Rega and his colleague were confronted by the two Americans after a drug deal went wrong, a court heard on Saturday.

The Americans, said to have been in Rome on holiday, allegedly went to Trastevere, a neighbourhood popular with tourists and young people, late on Thursday night in search of cocaine. There they met a middleman, who took them to a drug dealer, who sold them aspirin instead of the requested drug, the court heard. In retaliation, police said, they snatched the middleman’s rucksack, which contained his mobile phone, and fled before demanding a cash ransom and cocaine to return the bag.

The crime scene after Mario Cerciello Rega was stabbed on Friday. Photograph: Vincenzo Pinto/AFP/Getty Images

The middleman was able to contact the pair after calling his phone, and arranged to meet them in the nearby Prati area, the court heard. But he had also contacted police after reporting the theft, and the two officers went to the site.