The Champions League tie between Atalanta and Valencia last month accelerated the spread of the coronavirus in Lombardy, Italian authorities fear.
The Bergamo club staged the first leg of the quarter-final in Milan's San Siro Stadium on February 19 – two days before the first official cases of Covid-19 in Italy.
There were 45,792 fans at the match including 2,500 Spanish fans.
Around 35,000 Atalanta fans made the 35-mile journey from Bergamo to Milan by train, car and coach.
Since then, Lombardy and Valencia have become European coronavirus hotspots and the Champions League match has been labelled “Match Zero” after the search for “Patient Zero” who brought the virus to Italy.
“I think that the pandemic had started before in the countryside,” said Massimo Galli of the Sacco Hospital in Milan.
“But having tens of thousands of people concentrated in the same area because of this match means could certainly have been an important vector of contagion.”
Francesco Le Foche, an immunologist at the Umberto I hospital in Rome, added: “It's been a month since this match. The timing is relevant.”
The Italian army has been called in to take away the dead from Bergamo.
Italy coach Roberto Mancini said: “The footage of the military convoy that carried the coffins away from Bergamo was a punch in the face. Nobody was ready for this hell.”
More than 3,000 Atletico Madrid fans travelled to Liverpool for a Champions League tie on March 11 – the last big match in England before the shutdown.
The number of cases of coronavirus worldwide has reached 340,000, with nearly 15,000 deaths around the globe.
