Italian news reports say that Mario Scaramella was detained at Naples airport as he returned from London.
Rome prosecutors have been investigating him for arms trafficking and violating state secrets.
Scaramella met with Litvinenko in London the day the Russian fell ill. Litvinenko died of radiation poisoning last month.
Aleksandr Litvinenko: A Timeline
CLOAK AND DAGGER: A timeline of a murder case that unraveled after Andrei Litvinenko, a former Russian security officer and vocal Kremlin critic, dies on November 23, 2006, of poisoning by radioactive isotope polonium-210.
In a deathbed letter, Litvinenko blames Russian President Vladimir Putin for his death -- a claim Putin condemns.
Investigators center on two meetings in London that Litvinenko had described -- one in which he met with two Russians for tea in London, and another in which he met with a third Russian at a sushi bar.
Six months later, British prosecutors announce they have enough evidence to charge a Russian citizen -- one of the men who had met with Litvinenko for tea -- with the murder ...more...
MORE: Coverage in Russian from RFE/RL's Russian Service.