Unusual betting activity in the city of Frosinone before the club's Serie A defeat to Napoli on Saturday has led to an investigation being opened into a suspected manipulation of the game.

Three betting companies -- Intralot, Eurobet and Goldbet -- all noted large sums of money being placed on accumulator bets that involved one Frosinone player being sent off and Napoli winning the game.

Frosinone midfielder Mirko Gori was shown a red card after just 13 minutes of the game for kicking the ball against Napoli's Lorenzo Insigne as he lay on the ground after a foul.

With Napoli needing to win to finish second and Frosinone already relegated, a victory for the Azzurri -- which would have completed the accumulators -- at that stage was already extremely likely.

Napoli won the game 4-0 thanks to a Gonzalo Higuain hat trick.

According to La Gazzetta dello Sport, bets on Frosinone being given a red card were suspended before kick-off due to the unusual activity, which has been passed on to an office set up in the wake of previous match-fixing scandals to oversee a correct betting system.

The Journalistic Agency for Gaming (AGIMEG) reports that since betting on the red card for Frosinone was suspended due to the unusual activity, no winnings have yet been paid out until an investigation by the authorities is completed.