(By Denis Greenan). Turin, April 12 - The Informal Anarchist Federation (FAI) and the International Revolutionary Front (FRI) on Friday claimed responsibility for a letter bomb delivered to the La Stampa newspaper offices in Turin on Tuesday. FAI has staged a string of attacks in Italy and abroad over the years, while FRI is a smaller and more recently formed leftist extremist group which has apparently teamed up with it. In a letter to the Genoa daily Secolo XIX, the groups said that they had sent the device that was wired to explode. Three suspected FAI members were arrested in connection with the incident Thursday. The Italian newspaper offices were evacuated after police specialists determined the package was not a hoax, and contained explosives that were primed to blow when opened. Last May, members of FAI attacked and shot in the legs the CEO of the Ansaldo nuclear power company, Roberto Adinolfi, as he left his Genoa home to go to work - an attack which sparked fears Italy risked seeing a return of the political violence it endured in the 1970s. Italian anti-terrorist police investigating the group's message said that the letter bomb was 'dedicated' to two FAI members, Alfredo Cospito and Nicola Gai, who were arrested in September for the kneecapping of Adinolfi. Later on Friday several hooded protesters broke away from a larger group demonstrating in Turin and threw stones at the windows of the La Stampa newspaper offices and at staff members, injuring one man. An editor, photographer and driver from the publication were trapped in a car when several anarchist demonstrators began hurling stones, striking the driver on the wrist and arm. Police said that approximately 50 anarchists were protesting the arrests of the three FAI suspects in connection with the letter bomb. In their letter to Il Secolo, FAI said: "La Stampa is no more than a paper of the regime, so it is a target of our war against the state and society that sustains and legitimizes its existence every day". Anti-terrorist investigators said that they had "expected" a vendetta attempt by the group and that they were sure it "was not over". The most recent report from Italy's domestic intelligence service said that a wave of arrests of FAI operatives had put the group in "operational stasis". The biggest coup against both FAI and FRI came last June when Italian police arrested about a dozen suspected members of the groups. Eight people were arrested in Italy and two people already being detained abroad were also served arrest warrants. The arrested people were suspected of involvement in numerous attacks, including letter bombs sent in 2011 to Italian tax-collection agency Equitalia, Deutsche Bank in Frankfurt and the Greek Embassy in Paris. Interior Minister Anna Maria Cancellieri said at the time that the operation "represents an important achievement for the State against the anarchist-insurrectionist threat obtained thanks to the precious investigative work of the police and magistrates," said The minister added that she had called the police to congratulate them on a "brilliant operation". Her comments were echoed by the policeman in charge of the Perugia-based operation. "It is a response from the State to the anarchist-insurrectionist aggression," said Giampaolo Ganzer. He said that the investigation had uncovered the brains behind the attacks and the people who actually carried them out. Ganzer added that officers had seized material that could be used to make explosives during searches of a number of homes. The foreigners served warrants were a Swiss man already in jail in his homeland and a Spaniard being held in Germany. Ganzer said the investigation had uncovered strong links between Italian and Greek anarchist extremists. He said 24 people in addition to the 10 arrested were under investigation, including six Greeks who are already in custody.