Security services also concerned about foreign fighters from Syria after so-called Islamic State all but defeated there.
Italian police on Thursday arrested five Tunisians as part of what they said was a major operation against suspected supporters of Islamist terrorism in central and southern Italy.
The swoop is the latest in a series of similar raids this month, coming as Italy steps up the number of foreigners it expels on grounds of national security.
On Wednesday anti-terrorism police arrested an Italian citizen of Moroccan origin who they said was a so-called Islamic State sympathizer planning a truck attack.
"A vast anti-terrorism operation" was being carried out by special investigation forces based in Rome and the nearby town of Latina, police said in a statement on Thursday.
The five Tunisians detained are suspected of being part of a network connected to Anis Amri, the Tunisian who killed 12 people when he drove a truck into a Christmas market in Berlin in December 2016, judicial sources said.
Amri was shot dead by Italian police near Milan four days after the Berlin attack.
Prosecutors suspect the arrested Tunisians of crimes including planning international terrorism, falsifying documents and helping illegal immigration of radicalized Tunisians, the sources said.
However, they said there was no evidence the suspects participated directly in the Berlin truck attack - or that they were preparing any imminent attack in Italy.
Interior Minister Marco Minniti said on Wednesday the security threat to Italy from Islamic State supporters was higher than ever because more foreign fighters were attempting to return to Europe via Italy after the jihadist group suffered defeats in Syria and Iraq.
So far this year, 29 foreigners, mostly Muslims, have been expelled from Italy as suspected threats to national security.