One of the American teenagers charged with killing an Italian cop was “illegally blindfolded” before his interrogation, police admitted Sunday — as the newlywed officer was mourned by a crowd of thousands.

Finnegan Lee Elder, 19, confessed to stabbing plainclothed cop Mario Cerciello Rega, while his pal Gabriel Christian Natale-Hjorth, 18, admitted punching another officer in a bungled drug deal.

A photo showing Natale-Hjorth sitting in a Rome police station with his hands cuffed behind his back and his eyes covered by what appears to be a gray scarf was leaked to Italian media Sunday.

Carabinieri Commander Francesco Gargaro said the teen tourist was “illegally blindfolded… for a very few minutes, four or five” on Friday just before he was questioned in the case.

The Carabinieri police force and prosecutors are probing the photo since blindfolding a suspect is “illegal. It is not allowed,” Gargaro said. They’re also investigating who took the picture and how it was leaked.

The officer who put on the eye covering made a “mistake” but did so in order to prevent Natale-Hjorth from seeing documents related to the investigation, Gargaro added. The cop was transferred to a different unit.

Lawyers for the teens weren’t immediately available to comment to the Associated Press.

Meanwhile, mourners lined up outside a Rome chapel where the coffin of Rega, 35, sat ahead of his funeral, which will take place Monday in his hometown near Mount Vesuvius.

His wife, Rosa Maria Esilio, was seen pressing her hands against the coffin.

Also, flowers were also laid in front of the station where the officer — who had just returned from his honeymoon — was knifed eight times early Friday during an undercover operation.

The teens were brought to the stationhouse in handcuffs from their posh hotel near the Tiber River, where police say they were packing up and getting ready to skip town. Authorities found the large knife allegedly used to kill Cerciello Rega hidden behind a ceiling panel.

The tragic incident began when the pair, classmates in ritzy Mill Valley, tried to buy cocaine — but realized their dealer had sold them $100 worth of aspirin powder instead, police and local media said.

Furious, the teens allegedly swiped their dealer’s bag and cellphone and demanded cash and a gram of cocaine to return the items.

The robbery victim tipped the cops off, and they showed up to arrest the teens, when a brawl broke out and one of the boys pulled out a knife. It’s unclear if Cerciello Rega and his partners identified themselves as officers.

Elder’s family released a statement expressing their “deepest condolences to the grieving family and community that loved Brigadier Cerciello Rega.”

The officer, who had been married for just 43 days when he was killed, is being hailed as a hero across Italy. Some politicians worry the leaked photo might hinder the prosecution of the teens.

“To those who complain about the arrest of a prisoner, I remember that the only victim to cry for is a man, a son, a 35-year-old husband, a #Carabiniere, a servant of the Fatherland died in service at the hands of people who, if guilty, deserve only life imprisonment,” tweeted Deputy Prime Minister Matteo Salvini Sunday, along with an image of the blindfolded teen.

Salvini and Italy’s other deputy prime minister, Luidi Di Maio, are due to attend the officer’s funeral, which will take place in the church where he was married.

With Post wires