Fabrizio Piscitelli, known as “Diabolik”, was ambushed at Acqueducs, not far from Cinecitta, in the suburbs of Rome.
His attacker reportedly approached him from behind and shot him in the left side of his head.
Media reports claimed that Piscitelli, 53, had recently been the subject of a drugs investigation and that his murder was linked to rivals keen to settle scores.
“Piscitelli had lots of enemies and lots of business with various criminal groups,” a police source told Italian news agency ANSA.
“He was a central figure with links to various criminal environments, including Albanian ones.”
ANSA reported that Piscitelli had a criminal record for drugs trafficking and that police had seized €2 million in assets from him in 2016.
Piscitelli led the so-called Irriducibili – Lazio’s hardcore “ultra” supporters, seen as a far-right group, notorious for their anti-Semitic gestures, Celtic crosses, monkey chants and fascist salutes.
In August 2018, the group tried to ban women from sitting in the front rows of the stadium.
The club was fined €50,000 last year after supporters displayed stickers showing Holocaust victim Anne Frank in a Roma shirt along with anti-Semitic messages.
Lazio ultra fans hung a banner in front of the Colosseum on Wednesday night, which read: “DIABLO LIVES! IRR.”
Flowers were also laid at the murder scene.