Paolo Gabriele, under arrest by Vatican police after being accused of leaking the pontiff's confidential correspondence, will "cooperate fully" with Vatican magistrates investigating the leaks "to ascertain the truth," Gabriele lawyer Carlo Fusco said.
Vatican observers say Gabriele's pledge could mean high-ranking prelates will be named in the investigation into leaks of the pope's confidential correspondence, the British newspaper The Guardian reported.
Italian media report a cardinal is widely suspected of playing a major role in the "Vatileaks" scandal.
Vatican spokesman the Rev. Federico Lombardi Monday dismissed the reports as "unfounded speculations" harming the church."
"There is no cardinal under suspicion," he said, adding, "It is painful to see such a negative image" emerge of the Roman Catholic Church in Rome.
The scandals "put trust in the church and the Holy See to the test," he told reporters. "That's why we must confront them directly and not hide."
Vatican insiders said they were baffled that Gabriele, 46, could have been involved in the scandal, involving leaks of dozens of documents, The Guardian said.
They said he was always considered extremely loyal to Benedict and to predecessor Pope John Paul II, whom he briefly served, and said they believed he was a scapegoat.
Gabriel's arrest came a day before Vatican Bank President Ettore Gotti Tedeschi was dismissed, after the bank board suggested he might have leaked documents amid conflicts over how to bring the secretive bank in line with international transparency standards.
Several Italian newspapers carried an interview with an anonymous whistle-blower who said "the real brains behind [the leaks] are cardinals" and Gabriel is "just a delivery boy that somebody wants to set up."
"It's open warfare, with everyone against everyone else," he said, as translated by the British newspaper The Daily Telegraph.
Many leaked documents found their way to Italian journalist Gianluigi Nuzzi, whose book, "Your Holiness: The Secret Papers of Benedict XVI," was published days before Gotti Tedeschi's ouster.
The Vatican condemned the book's publication as "a criminal act" because confidential documents were "stolen" from the pope in a violation of his privacy.
Italian news media suggested the leaks were part of a power struggle designed to discredit Cardinal Tarcisio Bertone, Benedict's right-hand man and the Vatican's secretary of state.
Many critics, including some inside the Vatican, see him as a poor administrator who as the Vatican's chief executive officer has struggled to manage the papacy of a pope with little interest in day-to-day affairs of state, The New York Times said.
Nuzzi says in his book he was aided by an informer named "Maria."
He cites a 2011 letter to the pope from Archbishop Carlo Maria Vigano, then the deputy governor of Vatican City, who begged not to be transferred for having exposed alleged Vatican corruption.
Nevertheless, Bertone named Vigano the Vatican's U.S. ambassador and transferred him to Washington, where he has had to contend with multimillion-dollar lawsuits against U.S. dioceses over the church's sexual abuse scandal, the book said.
Other leaked documents cited in the book indicate that just before Christmas 2011 Italian TV newscaster Bruno Vespa sent a check for $12,500 to the pope's private secretary, Msgr. Georg Ganswein -- calling it "a small sum at the disposal of the pope's charity" -- and asked for a private audience with the pope in return.
It is not clear whether the request was granted, but Ganswein made a note to discuss it with Benedict, the book said.