Arifil Azim Putra Norjai, a 19-year-old post-secondary student who was detained in April, is the first known self-radicalised Singaporean to harbour the intention to carry out violent attacks in Singapore, said the Ministry of Home Affairs (MHA).
Investigations showed that he had plans to join the Islamic State (IS) terrorist group and that his radicalisation began around 2013 when he started viewing terrorist propaganda online, the ministry said.
It said Arifil then grew to support the radical ideology and violent tactics of IS, and befriended individuals online who he thought could help him join the terror group.
He also revealed that if he was unable to join IS in Syria, he intended to carry out violent attacks in Singapore, the ministry said.
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He had put "considerable thought" into how he would attack key facilities and assassinate government leaders.
If he was unable to carry out these plans, Arifil planned to carry out attacks in public places "in order to strike fear within our society", using "easily available" weapons such as knives, the ministry said.
The ministry said that another radicalised Singaporean youth, aged 17, was arrested in May under the ISA for further investigations into the extent of his radicalisation.
Deputy Prime Minister and Minister for Home Affairs, Teo Chee Hean has said Singapore faces real threats from radicalisation, similar to other countries.
He said that attacks can still happen, even if a country is already on high alert.