The suicide bomber at the Medan Police headquarters in North Sumatra on Wednesday is believed to have been active in a Telegram chat group belonging to homegrown terror group Jamaah Ansharud Daulah (JAD) and to have pledged allegiance to the new leader of the Islamic State (IS) group before carrying out the attack.

The revelation has highlighted the terrorist group's lasting appeal to vulnerable young Indonesians, despite extensive arrests by the nation's counterterrorism squad, military defeats in the Middle East and the recent death of its first caliph, Abu Bakr al-Baghdadi.

Baghdadi was killed in a raid by United States special forces in northwestern Syria in late October. He was replaced by Abu Ibrahim al-Hashemi al-Quraishi.

Three organizations that regularly monitor social media conversations of militant groups — the Terrorism Research and Analysis Consortium (TRAC), the SITE Intelligence Group and the International Association for Counterterrorism and Security Professionals Center for Security Studies (IACSP) — have confirmed that IS supporters in Indonesia celebrated the Medan attack and identified the perpetrator as one of their own.

"Islamic State East Asia/Jamaah Ansharut Daulah supporters claim on Telegram that suicide bomber in Medan, North Sumatra, Indonesia, pledged to new caliph last night before the attack," tweeted TRAC.

TRAC is a group that provides researchers in terrorism studies, international relations and other relevant fields with data and analysis about — among other things — terror attacks throughout the world.

The US-based SITE Intelligence Group said the bomber — identified by the police as 24-year-old Rabbial Muslim Nasution — was active on the cloud-based instant messaging application and that local JAD supporters were calling for more attacks.

IACSP Indonesia's country director Rakyan Adi Brata confirmed the claims made by TRAC and the SITE Intelligence Group.

His team, which monitors the activities of certain Telegram chat-groups where "IS doctrine was actively spread", found a conversation clarifying the identity of the suicide bomber. A screenshot of the conversation was obtained by The Jakarta Post on Thursday. It featured a message that said the suicide bomber was a member of the Medan cell and had pledged allegiance to IS the night before the attack.

The group's members, Rakyan said, celebrated the attack, which killed no one but the perpetrator and injured at least six people. They "congratulated" him for "finally finding his way to heaven as a reward for committing jihad".

Rakyan said that the IS group remained influential in Indonesia despite the government's crackdown on its sympathizers.

Fight against terrorism: Heavily armed Indonesian Military personnel stand guard in front of the house of a suicide bomber involved in a recent attack on the Medan Police headquarters as officers of the National Police’s Densus 88 counterterrorism squad and North Sumatra Police search the interior on Thursday.(JP/Apriadi Gunawan)

IS ideology is alive and well on social media, particularly on Telegram. IACSP found at least 12 chat groups whose members actively spread IS ideology. Each of the groups consisted of hundreds of members.

He urged the authorities to block Telegram in Indonesia to limit the group's reach.

Telegram, a highly encrypted messaging service, has been widely used by terrorist organizations to spread propaganda and recruit members.

The browser version of the messaging app was blocked by the government in July 2017 for its rampant use among local terrorists. However, after Telegram reportedly agreed to create a system to filter out content related to terrorism and radicalism, the government restored full access to the app in August the same year.

Militants, however, could still freely use the application.

"The threat of Telegram use is bigger than its benefit here," Rakyan argued.

The North Sumatra Police said they were still investigating the claim that the suspect pledged allegiance to the new IS leader before carrying out the attack. They were also investigating whether he was one of the JAD members active on Telegram.

"We are still collecting information about the allegation," North Sumatra Police spokesman Tatan Dirsan Atmaja said.

The police said they were hunting a cleric believed to have influenced the suicide bomber.

Investigators had established the identity of the cleric, deputy North Sumatra Police chief Brig. Gen. Mardiaz Kusin Dwihananto said. Identified only as SA, he is alleged to be the spiritual leader who influenced the suicide bomber to commit the crime.

“The person we are hunting is believed to be the imam of the suicide bomber at the Medan Police headquarters. We have information on his identity,” he said at the Bhayangkara Hospital after visiting injured victims on Wednesday evening.

He added that the police had searched the house of the cleric in the Belawan area but that he was nowhere to be found.

The police also said the Medan suicide bomber might have been exposed to terrorist ideology through his wife, DA, who had interacted with a female terrorist convict currently detained in Medan.

While stopping short of confirming if Rabbial was a JAD member, the police said that DA had actively communicated with JAD cells in other parts of the country through social media.

"The network seems to be planning an attack on Bali," National Police spokesperson Brig. Gen. Dedi Prasetyo said, adding that DA had been arrested by the police's Densus 88 counterterrorism squad in her home in Medan.

A number of security analysts suspect that the Medan suicide bombing was executed by IS supporters as an act of revenge for the death of Baghdadi, who had declared himself caliph of all Muslims.

“The purpose was to avenge the death of IS leader [Baghdadi],” said Mohamad Nurzzaman, the head of the strategic study division at the country’s largest Islamic group Nahdlatul Ulama (NU) youth organization as quoted by NU Online.

The government has, in recent years, adopted a hard-line approach to counterterrorism by prosecuting suspected terror group members.

Observers have placed the number of arrests between 2015 and 2019 in the thousands. The greatest number of arrests was during 2018, when 390 people were detained. National Police data shows that at least 143 people have been arrested from the beginning of this year to August, 33 of whom have been released. Most of those arrested were linked to JAD.