The Vatican says it has detained a person suspected of leaking a series of confidential documents and letters to the media.
Reports citing unnamed sources said he was the Pope's personal butler.
The "Vatileaks" scandal, as it is known, has enraged the Holy See. The leaks have revealed alleged corruption, mismanagement and internal conflicts.
The Vatican said the person detained was being questioned by Vatican magistrates.
Last month, Pope Benedict XVI set up a special commission of cardinals to investigate the leaks.
"The inquiry carried out by Vatican police... allowed them to identify someone in possession of confidential documents," Vatican spokesman Federico Lombardi told journalists.
"This person is currently being questioned."
'Poison pen' memos
Italian media have named the arrested man as Paolo Gabriele, a personal butler and assistant to the Pope and one of very few laymen to have access to the Pope's private apartments.
Among the select number who are allowed to enter the Pope's private study include four nuns and two secretaries, as well as the butler.
Italian media reports said that a stash of confidential documents had been discovered at his flat, within the walls of the Vatican.
The news of the arrest comes just a day after the president of the Vatican bank, Ettore Gotti Tedeschi, was ousted by its board.
The official reason for his departure was his failure to fulfil the "primary functions of his office", the Vatican has said.
But, according to reports, he was also suspected of being involved in the leaking of the documents.
Mr Tedeschi said he had been punished for his attempt to make the bank more open.
"I have paid for my transparency," he told Reuters.
The Vatileaks scandal has filled Italian media - dominating the columns of Italian newspapers and filling TV programmes and magazines.
The BBC's David Willey, in Rome, says the leak of a string of highly sensitive internal documents from inside the Vatican's Secretariat of State, including personal letters to Pope Benedict XVI, has been an evident embarrassment to the Pope, prompting the rare investigation.
The leaked documents include a letter to Pope Benedict by the Vatican's current ambassador to Washington alleging cronyism, nepotism and corruption among the administrators of Vatican City.
Others concern "poison pen" memos criticising Cardinal Tarcisio Bertone, the pope's number two, and the reporting of suspicious payments by the Vatican Bank.
Our correspondent explains that if the person detained is a layman he would be put on trial in Italy, under Italian criminal law.
But if he is a churchman, he could be tried for breaching ecclesiastical law by a Vatican tribunal, though this would be extremely rare.