"The attackers specifically and repeatedly targeted Prime Minister Lee Hsien Loong's personal particulars and information on his outpatient dispensed medicines," the Ministry of Communications and Information and the Ministry of Health said in a joint statement.
They said the cyberattack on July 4 was "deliberate, targeted and well-planned" and not the work of casual hackers or criminal gangs. Police investigations were ongoing, and a review of the SingHealth system will be conducted and affected patients will be informed.
Channel NewsAsia reported that investigators had already determined who was behind the attack. But in a news conference with local media, David Koh, chief executive of the Cyber Security Agency of Singapore, declined to discuss the perpetrators for security reasons.
The attack on the SingHealth database is believed to be the most serious breach of personal data in Singapore's history.
Around 1.5 million people who visited outpatient clinics from May, 1, 2015, to July 4 this year had their personal data accessed and copied, including names, identification card numbers, addresses, race, gender and dates of birth. Of that total, 160,000 also had their records of dispensed medicines copied too.
Officials said the patients' information was not amended or deleted. And the hackers did not have access to other records, such as diagnosis documents, test results or doctors' notes, the statement said.
Lee, who has been Singapore's prime minister since 2004, has been treated for intermediate-grade malignant lymphoma and prostate cancer. He underwent surgery to remove his prostate gland in 2015 and was subsequently given the all-clear by doctors.
"I don't know what the attackers were hoping to find. Perhaps they were hunting for some dark state secret, or at least something to embarrass me. If so, they would have been disappointed," Lee said in a Facebook post on Friday.
"My medication data is not something I would ordinarily tell people about, but there is nothing alarming in it," he said.
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