The police here said their investigation began when a public official alerted them after receiving a suspicious email.
Lawyers for the siblings, who moved between residences in London and Rome and were arrested on Tuesday, dismissed the accusations as unfounded.
But even as details of the investigation — called Eye Pyramid after the name of the malware used to hack into thousands of accounts — were revealed by the Italian news media, the motives remained murky, while criticism mounted over the relative ease with which confidential accounts were breached.
“There is no doubt that this case shows how the country’s cybersecurity system is still lagging behind,” Antonello Soro, president of the Italian Data Protection Authority, told an Italian radio station on Wednesday. The breach “is the tip of the iceberg of the fragility of the system,” he said. Security measures have not kept up with the increasing risks, he said, adding that “cyberattacks have been growing by 30 percent a year.”
Investigators said in a statement that the Occhioneros had managed to infiltrate the accounts of more than 18,000 individuals, “allowing the massive theft of contents.”
The accounts belonged to “a galaxy of individuals” including politicians, in current and past governments, and other people “of national importance,” the statement said. It is unclear whether any money was stolen in the cyberattacks.
The Occhioneros used a data network based in the United States as “a way to hide the origins and lose track of the delinquent activities,” said Ivano Gabrielli, an official with the Italian cyberpolice unit that investigated the case. He noted that Ms. Occhionero had dual citizenship and had lived in the United States.
An Italian request to examine the servers in the United States has been formally submitted, he said, adding that new developments were likely to emerge once the data was made available.
The Italian police have focused their investigation on the brother and sister, who were well known in the country’s financial circles for their work on computer-based investment tools. For now, Mr. Gabrielli said, “there is no evidence of the involvement of other people.”
But cybercrime analysts noted that whatever information the Occhioneros were gathering had to be interesting to someone to have any value.
“What’s still unclear is who had ultimate access to the information, whether it was gathered for the Italian market, or whether it was to be delivered outside,” said Raffaele Marchetti, coordinator of the digital revolution and cybersecurity courses at Luiss Guido Carli University in Rome.
He said it was unlikely that the siblings were acting alone. “That kind of information is useless unless you have an institutional or financial contact that has some interest in having it,” Mr. Marchetti said. “Them, alone, isolated without contacts is an unreasonable story. We should expect more worrying information to emerge, who actually took advantage of this information and who backed, supported or gave coverage to these operations.”
Italian observers noted that intelligence gathering for an advantage in, say, business or politics was not a new phenomenon here. “In the service of who or what and serving what end?” asked the columnist Guido Gentili in an editorial on Wednesday in the newspaper Il Sole 24 Ore. “The investigations will tell us but the impression remains that Italy once again is displaying, at the nexus of institutions, politics and business, one of its worst aspects: that of compiling dossiers, unauthorized surveillance and illegal collecting of private personal and professional information.”
Italian news media also noted Mr. Occhionero’s affiliation with Freemasonry. Several of the accounts that were violated belonged to fellow masons, and the hacking could have been motivated by an ambition to acquire positions within the organization, some newspapers suggested.
Some analysts went further, offering comparisons with the Propaganda Due, or P2, scandal involving a masonic lodge that in the 1970s gathered so much secret information and recruited so many top Italians officials that it was referred to as a “state within a state.”
Stefano Bisi, who is Grand Master of the Grand Orient of Italy, the country’s principal Masonic organization, said that “a lot of water had passed under the bridge” since the P2 scandal, and that in fact “masons were the victims” of Mr. Occhionero’s cyberattacks. Mr. Bisi was among those whose computers were hacked, but he said he could not imagine what Mr. Occhionero “could have been looking for.”
Mr. Occhionero denies any wrongdoing, his lawyer, Stefano Parretta, said Wednesday. He explained that his client had servers abroad “for business reasons,” and that the large box of files found in Mr. Occhionero’s garage were simply his company’s accounting books.
Ms. Occhionero’s lawyer, Roberto Bottachiari, told the Italian news agency ANSA that she “didn’t even know how to use a computer.”
Mr. Marchetti, the cybersecurity expert, said, “The idea of gathering information has happened in Italy many times before.” And on every occasion, “it highlights the dark side of that kind of world,” he said. “It’s worrying.”