Police said on Thursday that the man was arrested "for possession of material which could be used for making explosives".
The arrest followed a police search of the man's apartment in Viterbo, a city of around 70,000 people in the central Lazio region, where they found material used to create a pipe bomb.
This included powder, glue, and small coins, and police also found a rifle, two air pistols and other weapons including a knuckle-duster.
He was arrested and charged on Monday, but police only made a public statement about the arrest on Thursday after a judge extended the detention order.
The investigation was prompted by the American intelligence and security service, which noted posts on the Italian's social media pages praising New York terror suspect Sayfullo Saipov..
Saipov is accused of carrying out a truck attack in the US city on October 31st last year, in which eight people were killed. He was arrested immediately after the attack and told investigators he was inspired by propaganda from the terror group Isis, though he pleaded not guilty to charges of murder and terrorism.
Before the FBI tip-off, the Viterbo man, reportedly of Latvian origin, was not known to police or security services for extremist views.