Investigators said today four letter bombs delivered in four days to top European Union officials and EU bodies involved in crime-fighting came from Bologna in Italy.
Police were investigating links between the letter bombs and Italian anarchists. EU headquarters said it had tightened some security measures but was taking the threats in its stride.
An explosive parcel was sent to EU Commission President Romano Prodi on Saturday and was followed two days later by one to European Central Bank chief Mr Jean-Claude Trichet in Frankfurt.
A third package was intercepted at the EU police agency, Europol, in The Hague yesterday and a fourth was sent today to another EU institution called Eurojust. It did not explode.
German public prosecutors investigating the Frankfurt package said they were looking into links to an Italian anarchist group.
An Italian judicial source from Bologna said the Eurojust letter bomb also came from the northern Italian city.
Italian newspaper la Repubblica said Italian police had a list of 250 names from across Europe, but no proof of who sent the letters.
Those anarchists are believed to fall under an umbrella group called "Euroopposizione", founded earlier this year. Its manifesto says it is against "Europe's masters, their war, their peace, their repression, their control", la Repubblica reported.
But terror experts said such devices can just as easily be sent by a small group, a criminal gang or a loner.
The letter bomb sent to Mr Prodi exploded in his hands but he was unhurt.
The letter to Mr Trichet was intercepted and the bank's executive board held its regular weekly meeting today. The ECB would not comment on any extra security steps.