The four people that were arrested on Saturday were already on the radar of Densus 88. When the woman, with initials DYN, posted a package to her parents at a local post office, Densus 88 intercepted a letter (within the package) that contained information about the planned attack. She wrote that she was ready to commit amaliyah (code for a suicide bomb attack). When police raided her rented room in Bekasi (about one hour from Jakarta), they found a high-explosive bomb in her bag.

These people can probably be linked to a terror cell based in Solo (Central Java), led by IS fighter Bahrun Naim. Just outside Solo police arrested Abu Izzah, member of this cell, who is believed to have made the bomb that was brought to Bekasi. Bahrun Naim may also be the mastermind behind the January attack in Jakarta when four civilians were killed in Central Jakarta.

Besides this Solo cell, authorities are concerned that there is an increasing amount of radical Muslims being recruited for terrorist purposes in Indonesia. More alarmingly, IS wants to base themselves in the southern part of the Philippines, close to Indonesia's islands Kalimantan, Sulawesi and the Moluccas (in this area border control is weak). It is believed that hundreds of Indonesians already traveled to Syria to fight with IS.

Indonesian authorities already stepped up security measures ahead of Christmas and New Year celebrations.