Indonesian terrorists believed to have returned from fighting with the Islamic State (ISIS) group in Syria are suspected of being behind an attempted chlorine bomb attack in a shopping mall last month, police said Wednesday.

The homemade device – made up of several bottles and a detonator – was discovered in the mall south of Jakarta after it failed to go off properly. 

Police said it was the first such attack ever attempted in Indonesia.

National Police Inspector General, Tito Karnavian, said the use of the chlorine resembled tactics employed by ISIS jihadists, who have taken over a vast swath of territory in Syria and Iraq.

“It really surprised us,” said the former commander of the police’s elite counter-terror unit.

“This is a signature of ISIS,” he added, referring to the jihadists by an alternative name. “It is connected to a group likely already returned from Syria.”

He said police were pursuing “very good leads” into the bomb attempt but would not reveal further details. Exposure to chlorine gas causes intense irritation to the eyes, skin and airways, and can be deadly.

ISIS has been accused of using chlorine, notably in a January 23 car bomb attack on Kurdish forces in Iraq. The Syrian regime has also been accused of carrying out chlorine gas attacks.

Indonesia, a hotbed of extremist violence in the past, has largely dismantled the Islamic terrorist networks responsible for a string of deadly
attacks throughout the country in the early 2000s.

But the rise of ISIS poses a new threat, with nearly 160 Indonesians confirmed by police as having left to join ISIS, and authorities worried about the potential for radicals to revive extremist groups on their return.

Indonesian anti-terror police this week arrested five men who allegedly arranged for a group of mostly women and children to try and enter Syria to join ISIS.