By Agustinus Beo Da Costa and Gayatri Suroyo

JAKARTA (Reuters) – Indonesian police killed a militant on Monday after he detonated a small bomb in the city of Bandung and authorities said they were investigating whether he had links to a radical network sympathetic to Islamic State.

Indonesia, an officially secular state with the world’s largest Muslim population, faces what many people fear is a growing threat from supporters of Islamic State.

Recent attacks by Islamic State sympathizers have mostly been poorly organized, but authorities believe about 400 Indonesians have left to join the militant group in Syria, and some could pose a more deadly threat if they came home.

The blast in the courtyard of a government office in Bandung, southeast of the capital Jakarta, did not cause any casualties and the bomber was shot by police after he ran into the building.

The militant had arrived at the office on a motorbike and placed his bomb, made with explosives packed into a pressure cooker, in the corner of the courtyard.

The attacker had demanded that an anti-terror police unit, Densus 88, release all detainees, according to provincial police chief Anton Charliyan.

The attacker may have been linked to Jemaah Ansharut Daulah (JAD), an umbrella organization on a U.S. State Department “terrorist” list that is estimated to have drawn hundreds of Islamic State sympathizers in Indonesia.

“There’s a possibility of JAD,” Charliyan said, when asked which group the militant belonged to.

The bomber had been jailed for three years after undertaking militant training in Aceh, a province on the northwest tip of Sumatra island, said national police spokesman Martinus Sitompul.

Indonesia had scored major successes tackling militancy inspired by the al Qaeda attacks on the United States in 2001. But there has been a resurgence of Islamist activity in recent years, some of it linked to the rise of Islamic State.

Authorities foiled at least 15 attacks in 2016 and made more than 150 arrests.

The most serious incident last year was in January when four suicide bombers and gunmen attacked a shopping area in central Jakarta.

Eight people, including all four attackers, were killed in the first attack in Indonesia claimed by Islamic State.

Militant attacks had been relatively rare in Bandung, about three hours away from Jakarta. Provincial police spokesman Yusri Yunus said the situation was “under control” after the bomber was killed.

(Additional reporting by Kanupriya Kapoor; Writing by Eveline Danubrata and Robert Birsel; Editing by Simon Cameron-Moore)