By Anthony Faiola / Stefano Pitrelli in Rome and Brian Murphy in Washington contributed to this report.

The arrest of two Vatican insiders on suspicion of leaking damaging internal documents signaled the return Monday of an unwelcome guest at the Holy See’s ancient gates: scandal.

Lucio Ángel Vallejo Balda, a 54-year-old senior Vatican bureaucrat, and Francesca Chaouqui, a 33-year-old Italian public relations maven known in some circles as “the pope’s lobbyist,” were taken into custody over the weekend.

The arrests came as the Vatican, which denounced their actions as a “grave betrayal” of Pope Francis’s trust, braced for the release of two potentially explosive books this week based on leaks that the Holy See appeared to link to the two suspects.

[Spilled secrets claim to show Vatican’s own sins]

The books — which reportedly contain fresh revelations about corruption and mismanagement in the Vatican and about challenges to Francis’s push for internal reform — include one by Italian journalist Gianluigi Nuzzi. Nuzzi’s 2012 book on a “Vatileaks” scandal rocked the papacy of Benedict XVI by detailing behind-the-scenes power struggles revealed in documents stolen by Pope Benedict’s butler. Nuzzi’s new book, according to his Italian publisher, is like a “crime novel” and even quotes from recordings of Francis chastising his “top brass.”

Francesca Immacolata Chaouqui, member of the special commission on economic reforms of the Vatican, poses for a portrait at her home in Rome on Sept. 16. (Franco Origlia/Getty Images)

At a time when the pontiff is grappling with deep divisions among his hierarchy over the direction of his papacy, the surprise weekend detentions could further expose the internal rifts between his ideological allies and factions that oppose his efforts to reform the church. They could also signal a new phase for a Vatican that has generally basked in the glow of good press under Francis and may now need to revert to damage-control mode again.

[Conservative dissent is brewing inside the Vatican]

On Monday, the publishers of both books said they would stick to their Thursday release dates despite the Vatican’s suggestion that it may pursue legal action.

“We haven’t snatched anything from anyone,” said Lorenzo Fazio, editorial director of Chiarelettere, Nuzzi’s Italian publisher. “This is the third book by Nuzzi on the Vatican, and he’s always based his work on incontrovertible documents. Now he’s talking about power and the Curia, sharing truths that cannot do anything but good for the need to reform expressed in many ways by Pope Francis.” The Roman Curia is the bureaucratic arm of the papacy.

The arrests also cast a light on the little-known office of the Vatican’s gendarmes, a sort of mini papal police force that conducted the months-long criminal investigation of the leaks and hauled both suspects in for questioning before arresting them. The gendarmerie corps has general autonomy and does not need papal approval to make arrests, although high-profile detentions and investigations often have high-level Vatican oversight.

Francis, according to a Vatican spokesman, the Rev. Federico Lombardi, was briefed on the arrests before they were carried out. Asked whether the pope would intervene in the cases, Lombardi said only that Francis “respects the competence of Vatican institutions.”

Both Vallejo and Chaouqui had been tapped for Francis’s reform commission, which aimed to overhaul the Curia. The Curia has faced charges of corruption, inefficiency and petty rivalries in the past.

Pope Francis delivers a blessing from the window of his studio overlooking St. Peter’s Square, at the Vatican, on Nov. 1. (Andrew Medichini/AP)

Vallejo, a Spanish monsignor and the second-in-command of a Vatican economic affairs office, was still being detained. Chaouqui was released after pledging to fully cooperate with the investigation, the Vatican said.

Vallejo belongs to a religious group tied to Opus Dei, a conservative Catholic organization. On Monday, Opus Dei’s Rome branch expressed “surprise and pain” over his arrest. “If the accusation is proven true, it will be particularly hurtful because of the damage done to the Church,” it said in a statement.

Vallejo was handpicked by Francis to help guide his bid to reform the Curia. Vallejo then helped choose Chaouqui for the reform commission, which wrapped up its business last year. Chaouqui drew criticism from some Vatican insiders after she posted racy photos of herself on Facebook and sent critical tweets, including one describing Tarcisio Bertone, the former Vatican secretary of state who was pushed aside by Francis, as “corrupt.”

[The pope’s powerful German ally]

In an interview with the Italian magazine L’Espresso in 2013, she was quoted as saying that she had access “to highly confidential documents” and mentioned a friendship with Nuzzi. In an interview with the Boston Globe last year, she said enemies of the pope were responsible for the criticisms against her.

“Unfortunately for the peace of mind of those enemies, we’re still here and the reform is still happening,” she said.

Both suspects face potential charges under a 2013 law passed after the Vatileaks scandal that made it illegal to disclose confidential Holy See documents and information. In announcing the extraordinary arrests, the Vatican seemed to foreshadow the release of any potentially damaging information and suggested that it may pursue legal action against the publishers and the authors.

“Publications of this kind do not contribute in any way to establish clarity and truth, but rather to create confusion and partial and tendentious interpretations,” the Vatican said in a statement. “We must absolutely avoid the mistake of thinking that this is a way to help the mission of the Pope.”

Nuzzi’s new book, “Merchants in the Temple,” draws on documents, interviews and recordings of Francis speaking in closed-door meetings, according to Chiarelettere, his publisher.

[A reading list to understand Francis]

The pope is quoted as dressing down top financial officials, saying “costs are out of control,” and demanding transparency after finding “unofficial budgets” that detailed funds allegedly misused by Vatican officials, according to the publisher. The book also looks at alleged attempts to sabotage Francis’s reforms, describes the apparently lavish lifestyles of some cardinals and claims to document the misuse of money collected in church offerings.

“If we don’t know how to safeguard our money, which can be seen, how can we safeguard the souls of the faithful, which cannot be seen?” Francis is quoted as saying at a meeting of his hierarchy, according to Chiarelettere. The book also purports to unveil the full explanation behind Benedict’s surprising decision to retire in 2013.

The second book, “Avarice: Documents Revealing Wealth, Scandals and Secrets of Francis’ Church” by Italian journalist Emiliano Fittipaldi of L’Espresso — the magazine whose Vatican leaks have included a draft of a papal encyclical on the environment in June — deals with financial and other scandals inside the Vatican.

In an interview Monday, Fittipaldi said that the two suspects were not his sources.

“I wish that [the arrested individuals] will prove to the gendarmes that they did not commit those crimes, of which I have no awareness except for what I read [in the Vatican] press release,” he said.

In a related probe, Italian media reported last week that Vatican forensic experts were investigating alleged tampering of the computer used by the church’s top auditor, Libero Milone, who was appointed a few months ago.