The Italian referees association (AIA) said it has been sent packages filled with bullets amid fan protests over the implementation of the video assistant referee (VAR) system.
AIA president Marcello Nicchi said on Thursday that he, vice president Narciso Pisacreta and referee selector Nicola Rizzoli all received the parcels.
VAR has been introduced in Serie A this season to mixed results and has previously been the subject of fan protests.
Nicchi, who said the police and the interior ministry are investigating the incidents, denounced a journalist who suggested fans "should shoot the referees."
He also criticised a complaint lodged by a consumer association over a Serie A referee's decisions in a game between Lazio and Torino that will result in the referee appearing in court.
"Can you imagine [Cristiano] Ronaldo or [Lionel] Messi, or any other player, ending up in court for having made a mistake or presumed error?" Nicchi asked.
Last month, an estimated 1,000 Lazio fans showed up outside the Italian football federation's offices to protest decisions by VAR, which will be used at the World Cup in Russia this summer.
AC Milan beat Lazio 2-1 on Jan. 28 after a VAR failed to recognise the ball first hit Patrick Cutrone's arm before he scored.
Nearly a month earlier, in a 0-0 draw with Inter Milan, Lazio was awarded a penalty but VAR revoked the decision as Ciro Immobile's cross hit Milan Skriniar's leg before bouncing up onto his arm, so the handball was declared involuntary.
"My appraisal of VAR is completely negative because it takes emotions out of football," Lazio coach Simone Inzaghi said at the time. "Now you don't even celebrate after a goal.
"We would have had an extra seven points this season without VAR."