Indonesian militants believed to have returned from fighting with the Takfiri group, ISIL in Syria are suspected of being behind an attempted chlorine bomb attack in a shopping mall last month, police said Wednesday.

Indonesian militants believed to have returned from fighting with the Takfiri group, ISIL 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 ISIL Takfiris.

"It really surprised us," said the former commander of the police's elite counter-terror unit.

"This is a signature of ISIL," he added, referring to the Takfiri militants 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.

ISIL has been accused of using chlorine, notably in a January 23 car bomb attack on Kurdish forces in Iraq.