The letter also threatened president of Italy's main private employers' association Emma Marcegaglia, former centre-left prime minister Massimo D'Alema, labour law professor Pietro Ichino, the Italian justice ministry's prisons department chief Franco Ionta and the head of the Lazio region's prisons watchdog, Angiolo Marroni.
Adnkronos tipped off Italian anti-terrorism police, who were investigating the letter. It was signed "For Communism, the Red Brigades, and the Galesi Centre for an Anti-imperialist Armed Movement."
Mario Galesi was a late new Red Brigades member and murder suspect killed in a shoot-out with police aboard a train in March, 2003.
D'Alema and fellow opposition MPs Luciano Violante and Piero Fassino were targeted by written threats and a bullet mailed to Adnkronos in early January from putative Italian anarchists.
That letter was signed by the Informal Anarchist Federation (FAI), and Italian group which claimed responsibility for a package bomb found outside the Greek embassy and defused on on 27 December 2010.
Nobody was injured in the incident. But two parcel bombs at the Chilean and Swiss embassies in the Italian capital on 23 December 2010 seriously injured two people who opened them.
Two letters and bullets threatening Violante and his wife were sent to Adnkronos last autumn and were signed by Italy's far-left Red Brigades.
Violante, a former parliament speaker and anti-terrorism judge, is an MP for the centre-left Democratic Party, as is Fassino, a former Italian foreign minister.
Ichino has been living under police protection for years after receiving various threats from the far-left new Red Brigades. He has worked on legislation to ease Italy's inflexible labour laws. The Red Brigades have murdered two Italian labour lawyers involved in the project since 1999.
Italy's new Red Brigades, are the successors to Italy's Marxist-Lenninist Red Brigades, which in the 1970s and 1980s carried out kidnappings and murders in Italy including that of former prime minister Aldo Moro in 1978.