Western intelligence agencies believe that northern Italy has been used as a key European logistics base for al-Qaeda activists, providing support for active units elsewhere.
For al-Qaeda, Italy is an ideal place - it is relatively easy to enter, has a well-established Arab immigrant population and good connections with the rest of Europe.
A member of the Italian parliamentary intelligence committee says they are watching more than 500 al-Qaeda sympathisers and believes that there maybe 15 sleeper cells in the country.
Italian investigators have used secret phone taps and tip-offs from the FBI to make several arrests and convictions.
Among those on trial is Ben Hani Lased who is charged with criminal association with intent to traffic in arms, explosives and chemical agents, and of receiving and trafficking false documents. Prosecutors accuse him of being the middleman for a terrorist network. He denies the charges.
In early 2002 four Tunisians were jailed in Italy after a Milan court found them guilty of terrorist-related offences. The prosecution alleged during the trial that they were connected with the al-Qaeda network.
One of the men, Essid Sami Ben Khemais, who was alleged to be the group's leader, was suspected of planning an attack on the US Embassy in Rome in January 2001.
Four Moroccans suspected of plotting to poison the water supplies in the area around the US embassy in Rome are also awaiting trial.