One of the American teens accused of killing a cop in Rome is now claiming he acted in self-defense when he stabbed his victim 11 times — with a 7-inch combat knife he brought with him from the US.

Italian Judge Chiara Gallo revealed Monday that suspect Finnegan Lee Elder says he only stabbed undercover cop Mario Cerciello Rega in a drug-deal-gone bad because he was being strangled.

But the judge — in denying bail for the suspect and his alleged accomplice — noted in a written memo that Elder, 19, didn’t have any marks on his neck.

Gallo said the viciousness of the crime also indicated a “total absence of self-control.

“Neither of the two suspects showed that they understood the gravity of the consequences of their conduct, displaying an excessive immaturity even with respect to their young age and the degree of violence’’ that occurred, the judge wrote, according to the Italian newspaper La Repubblica.

Elder and alleged cohort Gabriel Christian Natale-Hjorth, 18 — who attended the same high school in Mill Valley just north of San Francisco — had been drunk and trying to score cocaine when Rega’s murder went down early Friday, authorities said.

The teens were in Trastevere, a popular area known for its nightlife, when they shelled out $100 to a dealer for what they thought was coke — only to learn it was aspirin, police said.

The pair stole the dealer’s bag and cell phone in retaliation and demanded the equivalent of $112 plus a gram of coke to return the items, cops said.

The dealer secretly called the police, who set up a sting near the teens’ four-star Hotel Meridien Visconti.

A brawl eventually broke out between the suspects and undercover cops, including Rega, and Elder allegedly knifed the officer in the chest while Natale-Hjorth punched another policeman.

“Before falling to the ground, [Rega gasped], ‘They stabbed me,’ ” according to cops.

Elder had brought the murder weapon — described by police to La Repubblica as a World War II-era, US Marines-style combat knife with a leather handle — with him to Italy, authorities said.

The teens fled the slay scene but were quickly tracked down at their hotel, where the knife had already been washed and stashed in their room’s fake-tile ceiling, police said.

The pair also had tickets with them to fly back to the US, authorities said.

Italian officials said both young men have confessed to the crime. The pair could face up to life behind bars if convicted.

But the cops also took heat — after a photo surfaced over the weekend showing Natale-Hjorth being illegally blindfolded while in custody.

Police claimed that the suspect was only blindfolded “for a very few minutes, 4 or 5,’’ Friday.

Monday’s developments came as Rega’s funeral was held in his hometown of Somma Vesuviana near Naples — in the church where he was married six weeks ago.

The country’s military chaplain, Archbishop Santo Marciano, gave the eulogy.

He said Rega — whose casket was draped with the Italian flag — was well-known on the beat in historic Rome and was a deeply religious man who volunteered his own time to feed the homeless.

“He dedicated his life to the service of the weakest,’’ said Salvatore di Sarno, the mayor of Somma Vesuviana who personally knew the cop, to CBS.

With AP