Gabriel Christian Natale-Hjorth sits blindfolded in a police station in Rome. Natale-Hjorth, a suspect in the slaying of police officer Deputy Brigadier Mario Cerciello Rega, was blindfolded before he was interrogated, an Italian police commander said. Picture: Italian Carabinieri via AP

Rome - One of two US teenagers arrested for the killing of an Italian policeman was blindfolded and handcuffed while in police custody, according to a picture that has sparked controversy.

Gabriel Christian Natale-Hjorth, 18, was arrested with Finnegan Lee Elder, 19, following the fatal stabbing overnight into Friday of Carabinieri policeman Mario Cerciello Rega.

In the picture, which has been circulated widely on Italian and international media, Natale Horth is shown sitting inside a police station, head bowed, with a band covering his eyes.

The way the suspect was treated "is completely unjustified and will not be in any way tolerated," Carabinieri Rome Commander Francesco Gargaro told RAI public radio on Monday.

A criminal investigation is ongoing to determine who blindfolded Natale-Hjorth, who took the picture and who leaked it to the media, Gargaro confirmed.

Prime Minister Giuseppe Conte condemned the incident, and said the police officer suspected of mistreating the suspect has been removed from the case.

"To be clear: giving that treatment to a person deprived of liberty does not correspond with our legal principles and values, and actually constitutes a crime, or maybe two," Conte wrote on Facebook.

Far-right Interior Minister Matteo Salvini was more dismissive, writing on social media that "the only victim" in the affair is the dead officer.

Cerciello Rega was involved in a plain-clothes operation to retrieve a backpack that Natale-Hjorth and Elder had allegedly stolen from a drug dealer in retaliation for being sold fake cocaine.

The police officer, whose funeral was due to take place Monday, died of multiple stab wounds.

According to Italian police, Elder has confessed to the stabbing.