Two US tourists sentenced to life for stabbing an Italian police officer to death in 2019 begin their appeal Thursday, with their lawyers expected to argue that the verdict was biased.

Finnegan Elder, 22, and Gabriel Natale-Hjorth, 20, were convicted in May 2021 by a Rome court for the death of police officer Mario Cerciello Rega during a late-night encounter in July 2019 while the two were on summer holiday.

The stabbing of Cerciello with an 11-inch camping knife on a dark Rome street horrified Italy and led to an outpouring of public grief for the newly-wed officer, hailed as a national hero.

But the case revealed multiple examples of police misconduct.

And it hinged on whether the two then-teenagers knew the officers were police, with both sides offering very different versions about the moments leading up to the killing.

The evening had begun with a botched drug deal. The Americans later went to meet someone they expected to be the go-between on the failed deal -- but police showed up instead.

Cerciello's partner, Andrea Varriale, testified that the attack was unprovoked, coming immediately after the two plainclothes officers presented themselves as police.

But both Americans said they were jumped from behind by men they thought were drug dealers. They denied the officers had shown them their police badges.

Natale-Hjorth did not handle the murder weapon during the attack, scuffling instead with Varriale.

But he helped Elder hide the knife, and under Italian law faced the same homicide charge as his friend.

Finnegan Elder, 22, and Gabriel Natale-Hjorth, 20, were convicted in May 2021 by a Rome court
Finnegan Elder, 22, and Gabriel Natale-Hjorth, 20, were convicted in May 2021 by a Rome court POOL via AFP / Alberto PIZZOLI

Life in prison is Italy's stiffest penalty, and harsher than many given to mafia criminals or others who commit premeditated killings.

Lawyers for Elder criticised the sentence last year as "shameful for Italy".

In the appeal, lawyers plan to argue that the court ignored substantial evidence that Varriale -- the prosecution's star witness -- lied on the stand, and will also highlight what they say was a pattern of ignored protocol by police the night of the attack.

Among other inconsistencies, Varriale admitted having previously lied when he said after the attack that he had been armed, when he was actually without his gun.

Three other prosecution witnesses are being prosecuted for perjury.

The defence will also cite a court document explaining its reasoning for the conviction and sentences as evidence of bias towards law enforcement.

In that document published in July, the court criticised defence lawyers, saying they "mocked the conduct of the victims" during the trial, as they defended their clients "to the limits of permission and decency".

Elder's lawyers, Renato Borzone and Roberto Capra, said Tuesday that a "correct reading of the evidence" by the appeals court would result in a different outcome.

"The truth of what really happened that night is already in the documents collected during the first instance trial, you just need to want to see it," they wrote in a statement.

In a related proceeding, a trial began Tuesday against an officer who blindfolded Natale-Hjorth inside the police station following his arrest. A photograph of the handcuffed, blindfolded teenager went viral, sparking widespread criticism.

On Wednesday, La Corriere della Sera daily published messages from a group chat -- introduced as evidence in that trial -- that called for rough justice for Cerciello's killers, with one officer suggesting "we should dissolve them in acid".