Here is the leather-bound combat knife that Italian cops say an American teen used to fatally stab a police officer in Rome last week.

Police released a photo Tuesday of the murder weapon, which boasts a 7-inch blade.

California 19-year-old Finnegan Lee Elder is accused of bringing the hefty knife with him from the States to Italy and using it to kill undercover officer Mario Cerciello Rega during a murderous rage in a drug bust gone awry early Friday.

Elder’s former high school buddy, Gabriel Christian Natale-Hjorth, 18, is accused of punching Rega’s partner during the attack.

The photo of the knife surfaced as authorities revealed that Rega was unarmed when he was attacked — because he forgot his gun back at the police station.

“Cerciello did not have the weapon with him at the time of the attack but only the handcuffs. He had forgotten it,’’ Rome police commander Francesco Gargaro told reporters at a press conference, according to the Italian newspaper La Repubblica. “We found it in his locker in the barracks.”

“But it does not change’’ the case, the top cop said, because “like [his partner, Rega] would not have had time to react.”

Deputy prosecutor Nunzia E’lia said the two suspects reacted differently to hearing that the cop was dead.

Natale-Hjorth kept saying, “But is he really dead? Is he really dead?’’ while Elder “shed some tears,’’ the official said.