Daniele De Santis, a well-known AS Roma fan, has denied the shootings but remains under investigation for attempted murder because of other evidence, sources told Ansa.
The evidence is reportedly based on eye witnesses, who said De Santis, nicknamed Gastone, opened fire after being attacked by the Napoli fans.
The sources told Ansa that the gun was found by a woman behind a vase near the gates of a sporting club managed by De Santis. She is said to have thrown the gun in a rubbish bin before going to help him.
Meanwhile, Italian media reported on Monday that one of the Napoli supporters – 30-year-old Ciro Esposito, who is in a serious condition in hospital following an operation to remove a bullet from his back – was also arrested.
He was reportedly accused of fighting and provoking De Santis, who allegedly responded by shooting him and the other two Napoli fans.
But his lawyer, Angelo Pisani, told TGCOM24 that no notification of his arrest has been given.
De Santis, nicknamed Gastone, reportedly provoked the Napoli supporters using smoke bombs. When they reacted badly, he is said to have pulled out a gun and started shooting.
De Santis is from the hardline ‘Ultras’, an organized group which is responsible for much of the violence that blights football in Italy.
The incident happened ahead of the Italian Cup final between Fiorentina and Napoli.
The game, which started 45 minutes late, was won 3-1 by Napoli.