Indonesian authorities on Sunday arrested a suspected militant who threw an explosive device at a church in the eastern island of Borneo and injured four children.

Police in East Kalimantan province said the attacker had been imprisoned on terrorism charges in the past.

“A suspected low-impact bomb was thrown…Four children were injured and four motorcycles were damaged,” East Kalimantan police spokesman Fajar Setiawan said, adding police were looking into links with radical networks.

The attack took place at 10am in the parking lot of Oikumene church in the town of Samarinda.

Indonesia has seen a string of Islamic State-linked attacks this year, the biggest of which was a gun and bomb assault in the capital Jakarta that killed four people.

Authorities are concerned about a resurgence in radicalism and say there are hundreds of Islamic State sympathizers in the world’s biggest Muslim-majority nation.

President Joko Widodo said he had ordered an investigation into the attack.

“I have ordered the police chief to take firm action and thoroughly investigate the perpetrator,” Widodo said in a statement.

(Reporting by Agustinus Beo Da Costa, Writing by Kanupriya Kapoor; Editing by Elaine Hardcastle)