Two bomb-makers behind the device used in last week's suicide bombing of the police headquarters in Medan, North Sumatra, have been killed in a police raid, laying to rest initial suspicions that the attacker was a "lone wolf".
Investigations have shown that the suicide bomber was linked to local network Jemaah Ansharut Daulah (JAD), which is affiliated with the Islamic State in Iraq and Syria (ISIS).
The network was founded by terrorist ideologue Aman Abdurrahman, who is now on death row.
Identified by the police only by their initials KA and P, the bomb-makers had handmade firearms and resisted arrest during the raid last Saturday which left one policeman injured.
Three accomplices who were with them during the raid in Deli Serdang regency, about an hour from Medan, were arrested and are being interrogated.
Between last Wednesday's attack and last Saturday, the police have rounded up to 43 militants.
Twenty were nabbed in North Sumatra and neighbouring Aceh province, while another 22 were arrested in Java. One militant was detained in East Kalimantan, said the police.
The attack last Wednesday occurred at around 8.40am local time, 20 minutes after the bomber was seen on closed-circuit television walking into the police compound.
The young man, clad in a ride-hailing service jacket and carrying a big backpack, blew himself up in a carpark near a building where certificates of clearance, known as SKCK, were being processed.
The police later identified the perpetrator as RMN.
Six people - four police officers, one police employee and one civilian - were hurt in the blast, which also damaged several vehicles.
Eyewitnesses told local media that they heard a loud explosion and saw puffs of white smoke as they rushed out of the building.
Among the 20 people arrested was a senior militant who police identified as Yanto, alias Yasir, who is the JAD leader for North Sumatra and Aceh.
National police spokesman Dedi Prasetyo told a media briefing yesterday: "Yanto is a leader. He led others to pledge allegiance to the ISIS leader. He also led a paramilitary training on Mount Sibayak (in North Sumatra)."
North Sumatra police chief Agus Andrianto was quoted as telling reporters separately that the militants practised shooting arrows and horse riding at the paramilitary training camp.
But Kompas.com said General Agus was baffled by the horse-riding training and how that was connected to a terror attack plan.
Police investigations also found that Yanto communicated with a terrorist inmate in Medan prison, bought equipment for the Sibayak paramilitary training camp and materials for bomb-making.
The other militants rounded up by the police were those who either knew about last Wednesday's planned suicide bombing, or took part in the paramilitary training on Mount Sibayak.
Anti-terror analyst Adhe Bhakti told The Straits Times: "North Sumatra has various terrorist factions. There are quite a few pro-ISIS ones."
In March, the police found 300kg of explosives and more than 15 bombs in two houses belonging to a detained terrorist and an accomplice in northern Sumatra.
The materials - which included a land mine, pipe bomb and vest bomb - were mostly found in the Sibolga home of local militant cell ringleader Husain, alias Abu Hamzah, 30.
His wife later blew herself and her two-year-old child up when the police surrounded the house.
