Italian police on Monday arrested an Algerian male, 36, on suspicion of helping those behind the November 2015 Paris terror attacks which killed 130 people and wounded 350.
Athmane Touami is suspected of belonging to ISIS and forging documents for the attackers.
"The investigations have made it possible to ascertain ... the proximity of the suspect to radical jihadist environments, as well as his direct support to the authors of the terrorist attacks at the Bataclan theatre," police in the southern city of Bari said in a statement.
Mr Touami is suspected of being part of a cell operating in France and Belgium with his two brothers, Medhi and Lyes.
They are described as "forgers at the service of terrorist organisations" able to provide logistical support and other help, according to the detention order cited by Italian newspaper La Repubblica.
French investigating authorities have established that the perpetrators of the Paris attacks were in possession of 14 fake Belgian identity documents from the same manufacturer.
The detention order alleges that Mr Touami had been in contact with Belgian Abdelhamid Abaaoud, an ISIS extremist and mastermind of the Paris attacks, as well as Khalid Zerkani, the jihadist preacher in Brussels who recruited scores of young Muslims as jihadist fighters to Syria.
The forgeries enabled them "to carry out preparations for the attacks and in particular to rent flats, to travel around Europe to set up the terrorist cell, to withdraw money," the French authorities said.
The order also alleges Mr Touami and his brothers had contact with Amedy Coulibaly and Cherif Kouachi, the extremists behind the January 2015 Jewish supermarket and Charlie Hebdo attacks, respectively.
An AFP source close to the French investigation downplayed Mr Touami's involvement, however, calling his link to the perpetrators "tenuous".
Mr Touami is currently serving a two-year jail term in Bari for the possession of fake documents and was due to be released in June.