Mohammad Razif Yahya began studying at a religious school in Yemen six years ago, but as the security situation deteriorated, he volunteered to do armed sentry duties.
He also signed up for sniper training, which he put to use fighting the Houthis, a Shi'ite rebel group.
Razif, 27, and fellow student Amiruddin Sawir, 53, who was also involved in a firefight, were "prepared to kill and be killed as 'martyrs' in sectarian conflict", the Home Affairs Ministry said in announcing their detention.
News of their cases, as well as that of Mohamed Mohideen Mohamed Jais, 25, was greeted with dismay by Muslim community leaders yesterday.
The Islamic Religious Council of Singapore (Muis) said the arrests show that "there are institutions abroad that may masquerade as centres of Islamic learning, but which are actually involved in armed conflict and militant activities".
The Straits Times understands that Razif, Amiruddin and Mohideen all studied at Dar al-Hadith in Dammaj, Yemen. The school had a sizeable number of foreign students, many of whom were displaced by sectarian conflict.
Razif and Amiruddin returned home separately between April and June last year, and were arrested under the Internal Security Act in July. They were each issued a two-year order of detention in August.
Ustaz Muhammad Zahid Mohd Zin, vice-president of Islamic scholars and religious teachers association Pergas, said Yemen was one of the more popular destinations for Muslims who want to learn more about their religion abroad, alongside Egypt and Jordan.
While many enrol in recognised institutions, the three who fell foul of the law enrolled in a school that subscribed to a more puritanical interpretation of Islam, he said.
"They were not enrolled in the two institutions we have accredited and work with in Yemen," he added.
Dr Nazirudin Mohd Nasir, deputy director at the Office of the Mufti, urged Muslims in Singapore who wish to study Islam to approach only recognised teachers.
"For those who wish to study in foreign institutions, please consult Muis and we will provide guidance and the necessary support on the appropriate overseas institutions for Islamic studies," he said.
He also encouraged parents and family members to play a more active role in guiding loved ones to proper sources of Islamic learning.
"As a community, we must continue to be vigilant against extremist elements in our society," he said.
Ustaz Zahid noted that Pergas organises an annual voluntary pre-departure programme for madrasah graduates about to pursue higher Islamic education overseas.
The programme covers topics such as the security situation in the Middle East and steering clear of political activity and armed conflict.
"We know there are a few who go overseas by themselves to learn, but we can't know their exact number because they are not obliged to register with us," he said.
Pergas is now looking at making the programme compulsory, and will also urge students to keep a lookout for private Singaporean students so it can keep track of them.
Mr Abdul Halim Kader, president of community group Taman Bacaan, said it may be timely for the community to consider distance learning tie-ups with reputable Middle East institutions so students can obtain certification from such schools while studying here.
"Two decades ago, there was no problem sending your children to an Islamic university in the Middle East because it was peaceful, but the situation is very different today," he said. "Perhaps Muis should consider working with foreign universities to bring them here."
The Religious Rehabilitation Group urged people to call its helpline on 1800-774-7747 if they have questions about radicalisation.
