Anti-terrorism prosecutors based in the northern town of Brescia requested the man's detention, alleging he supplied false documents to various people.
Among them were two Syrians arrested last year at the airport of the northern town of Bergamo on suspicion they were close to the Islamic State group. The men had planned to board a flight to Malta.
Without giving details, police said today the Somali man was detained on suspicion he aided clandestine immigration and also helped persons involved in terrorist associations.