The prosecutor who placed the Italian interior minister, Matteo Salvini, under investigation over the illegal detention of migrants has received a death threat, sources have told the Guardian.

Believed to be the work of far-right extremists, an envelope containing a bullet was sent to the office of the chief prosecutor of the Sicilian city of Agrigento, Luigi Patronaggio, on Wednesday alongside the note: “You are under fire.”

The envelope was stamped with the symbol of a Gladius sword, a mark often used often by military groups close to far-right movements.

Patronaggio placed Salvini, who also leads the far-right League party, under investigation for the kidnapping and detention of 177 migrants after he refused to allow them to disembark the coastguard ship Ubaldo Diciotti last month.

The ship had been docked for six days at the Sicilian port of Catania as Salvini maintained a standoff with the EU in an attempt to push other member states to take in the migrants. The Catholic church, Ireland and Albania, which is not an EU state, eventually agreed to host the mostly Eritrean migrants.

Patronaggio had been attacked by far-right groups and supporters of Salvini on social media for this decision to investigate the minister. He declined to comment on the latest threat when contacted by the Guardian on Wednesday.

Patronaggio launched the investigation a few days after the Diciotti arrived in Catania, going onboard to question the passengers.

Magistrates travelled to Rome in late August to question members of Salvini’s staff and at least two high-ranking interior ministry officials before formally placing the minister under investigation. The investigation dossier was then passed to prosecutors in Palermo.

As news of the investigation emerged, Salvini responded on Facebook: “I heard prosecutors asked for my details. Here you go. I was born in Milan, 9 March 1973. I’m ready and proud to be arrested because I’m fighting to defend the Italian border.”

As stipulated by the Italian constitution, the accusations against Salvini will be put to parliamentarians, who will vote either for him to stand trial or for the proceedings to be stopped. If the case does reach trial and Salvini is convicted, he would face up to 10 years in jail.