In response to questions, Passionist Father Ciro Benedettini, assistant director of the Vatican press office, said the suspect was "under arrest." However, he declined to say if or where the person was being held.

Dozens of private letters to Pope Benedict and other confidential Vatican correspondence and reports, including encrypted cables from Vatican embassies around the world, were leaked to an Italian journalist, Gianluigi Nuzzi. He published the documents in a book, "Your Holiness," released May 17.

In a statement two days later, Father Lombardi called the publication of the letters for commercial gain a "criminal act" and said the Vatican would take legal action. The publication, he said, violated the right to privacy and the "freedom of correspondence" of Pope Benedict, the letter writers and the pope's closest collaborators.

In the book's introduction, Nuzzi said his main source for the texts said he was acting with a "small group" of Vatican insiders concerned about corruption and a thirst for power within the Vatican. According to the source, Nuzzi said, none of the people giving him documents knew who the others were.