On a Thursday evening three weeks ago, three men and a teenage boy from a boarding school in Bogor, West Java, got off a budget airline at Changi Airport.
They were dressed in T-shirts, jeans and casual jackets, and carried backpacks - not unlike many young Indonesian travellers.
But something about the group seemed odd to the undercover officer monitoring the passengers coming through the arrival gate at 9pm on Feb 18. His hunch proved right when they took the escalators a floor down to the immigration counters.
Mukhlis Khoirur Rofiq, 22, had a passport expiring the same day as that of his brother Muhammad Mufid Murtadho, who was just nine days away from his 15th birthday.
The brothers approached different counters. One followed Risno, 27, and the other, Untung Sugema Mardjuk, 48. The brothers could speak English, but their travel companions could not.
Singapore was not a launch pad for their travel - they came here just to get their passports stamped.
PROFESSOR ROHAN GUNARATNA, who heads the International Centre for Political Violence and Terrorism Research at the S. Rajaratnam School of International Studies, on the four Indonesians who used Singapore to build a travel footprint.
The Singapore authorities allowed them entry but continued to put them under surveillance.
Once they cleared customs, they took public transport to Woodlands Checkpoint. By midnight, they were on a bus that crossed the Causeway and was heading to Johor Baru. When it stopped at Larkin bus terminal in Johor, the four travellers went to a nearby prayer room to sleep.
The next morning, Friday, Feb 19, they boarded a bus and returned to Singapore.
Their unusual travel pattern prompted immigration officers to stop them at the passport counter and they were subsequently questioned by the Internal Security Department.
They were put on three separate ferries to Batam two days later on Feb 21, and handed over to Indonesia's counter-terrorism police.
BUILDING A TRAVEL FOOTPRINT
Mukhlis had booked a one-night stay for that Friday at a budget hotel in central Singapore on a popular Indonesian travel site. The group also had plane tickets to fly back to Jakarta on Saturday, Feb 20.
Unlike the two Indonesians who were detained on Nov 5 at the HarbourFront Ferry Terminal and were on their way to join Islamic State in Iraq and Syria (ISIS), this group was not bound for Syria immediately. They did not have enough money to head there yet.
Rather, in the first case of its kind detected here, the four wanted to build a travel footprint so that the authorities would regard them as legitimate travellers when they eventually had enough funds to head to the conflict zone.
"Singapore was not a launch pad for their travel - they came here just to get their passports stamped," said Professor Rohan Gunaratna, who heads the International Centre for Political Violence and Terrorism Research at Nanyang Technological University's S. Rajaratnam School of International Studies. "They have also admitted to the authorities in Indonesia that their intention was to travel to Syria and be part of ISIS."
Videos and material related to ISIS were found on the men's mobile phones, sources from intelligence agencies in the region familiar with the case said.
All four were from a school, the Pondok Pesantren Ibnu Mas'ud in Bogor, West Java.
Mukhlis taught religion and mathematics, while his younger brother was a student. Risno and Untung were cooks at the school, which had some 180 students.
Investigations by the Indonesian authorities found the school is associated with radical ideologue Aman Abdurrahman, who is in Nusakambangan prison in Central Java. Even from his cell, Aman has been influential in reaching out to ISIS supporters across the country.
He has also been in touch with Indonesian ISIS fighters in Syria and Iraq, many of whom are members of the South-east Asian unit Katibah Nusantara.
And Mukhlis, Mufid and their family were loyal supporters of that cause. Their father Armeidi was in a chat group with ISIS fighters and planned to sell his house and migrate to Syria with his family.
He and several of his family members took the bai'ah (oath of allegiance) to ISIS leader Abu Bakr al-Baghdadi in a ceremony in south Jakarta in 2014.
They believed that suicide bombing was justified, and were also prepared to kill other Muslims - because those who did not follow their ideology could be deemed disbelievers. The school also propagated these hardline ideas.
Mr Muh Taufiqurrohman, a senior researcher at Indonesia-based non-governmental organisation Centre for Radicalism and Deradicalisation Studies, told The Sunday Times that the school is one of at least three boarding schools to have emerged in recent years where ISIS supporters study or find work, and enrolled their children.
At least a dozen people from the school have travelled to Syria.
They include Mukhlis' elder brother Ghozian, a former treasurer at the school who left for Syria early this year with three others.
Ghozian had travelled through Singapore and Malaysia on transit to Thailand and then Turkey.
A senior Indonesian police source said Singapore's Changi Airport is a favoured stop for Indonesians travelling to fight in Syria given its proximity to home and flight connections. Yet, many also go undetected as transit passengers are not subject to immigration checks.
A former principal of the school, Abu Umar, also left for Syria with his wife and four children, and was last known to be in Mosul, Iraq.
The current principal - Mashadi, who is in his 30s - is said to be an ISIS supporter from Riau Islands.
Around 700 Indonesians are estimated to have travelled to Syria to fight, and the authorities in the region are concerned that when they return home, they will sow hatred.
More worrying, however, are those who never left but stayed in touch with Katibah fighters in Syria online. There are also those who are indoctrinated through schools like Ibnu Mas'ud.
The four who travelled to Singapore held hardline views - that suicide bombing was permissible, and killing other Muslims was all right if they did not subscribe to their beliefs. They also wanted to kill Shi'ites in Syria.
They did not meet people in Singapore, and Prof Rohan noted that the fact that they were detected shows the authorities are vigilant. There is also strong counter-terrorism cooperation between Indonesia, Malaysia and Singapore, he said.
When the four were sent back in three ferries - for security reasons - they were detained by Indonesian counter-terror police for questioning. The police recorded their statements, but had to let them go as there were no provisions under Indonesian law to detain them longer.
Mr Taufiqurrohman noted that other radicalised Indonesians, who were stopped before they could reach Syria, would still want to carry out attacks on Indonesian police as well as Shi'ite and other minority communities in Indonesia.
"The Indonesian security apparatus needs to monitor their activities closely, especially to find out with whom these four associate themselves," he said. "If they communicate with Indonesian ISIS fighters in Syria, they will pose a threat because they will continue to receive online bomb-making instructions, funding and orders to carry out terrorist attacks."
Even as the four were found out, it remains unclear just how many others have travelled to Singapore without being detected. Who else might have transited here on their way to Syria?
Observers like Prof Rohan say governments can be alert only up to a point. Much more remains to be done to step up vigilance and harden laws to tackle the terror threat.