Surabaya, Indonesia (CNN)The three families that carried out bombings this week in the Indonesian city of Surabaya knew each other and had all studied together in a Quran study group, according to authorities.
Families tied to Indonesian bombings studied religious texts together
The group met at the home of Dita Oepriarto, who led his wife and four children in an attack on three churches, East Java police Chief Mahfud Arifin told reporters Tuesday. The group is believed to have included other members besides those three families.
It's the first time in the history of Indonesian terrorism that suicide bombers have involved children. The church bombings were swiftly followed by other attacks, including one Monday that targeted a police station and was carried out in the company of the attackers' 7-year-old daughter. She survived after being flung from a motorbike, clear of the blast.
The attacks are thought to have been perpetrated in support of Jamaah Ansharut Daulah, an Indonesian jihadi group that supports ISIS. The children are believed to have had little or no contact with others outside their families and were shown jihadist videos by their parents, police said.
"They were not home-schooled," Arifin said in response to a question from journalists about their education. "They were indoctrinated."
Indonesia National Police Chief Tito Karnavian told reporters Monday that officers were working on the assumption that the attacks followed a directive from ISIS central command to avenge the imprisonment of former Jamaah Ansharut Daulah leaders.
Security remained tight around police buildings Tuesday in Surabaya, Indonesia's second-largest city. A gunfight broke out between counterterrorism police and suspected militants Tuesday, with authorities killing one of them, East Java police said. Thirteen suspects were arrested Sunday and Monday, while four others were killed after resisting arrest.
Elsewhere family and friends mourned the deaths of the 12 people killed in the church blasts.
At the Adi Jasa funeral home, the mother of two young victims was wheeled in on a stretcher from an ambulance to view the bodies of her sons.
Wenny Angelina lost her boys, Vincensius Evan Hudojo, 12, and Nathanael Ethan Hudojo, 8, in the blast at the Santa Maria Catholic Church. That attack was carried out by two teenagers, ages 16 and 18, the sons of Oepriarto.
Her brother Jo Prajoko told CNN he had dropped his sister and nephews at the church for Sunday prayers and was driving away when the bomb exploded.
"I looked back and saw a crack in the rear car window. I immediately got out of the car. ... I then saw the children laying on the ground. I saw some bodies torn apart. I helped Evan and Nathan (and) carried Evan first to ... the car. A security guard helped carry Nathan to the car."
Also among the dead were seven minors as well as an 18-year-old son from the three families who carried out the attacks.
A bomb prematurely exploded Sunday night at an apartment in the Surabaya suburb of Sidoarjo, killing a mother and daughter. The father was shot by police as he held a detonator. The family is survived by a 12-year-old son, who took his younger sisters to the hospital.
The three children are under the care of the police, the East Java police chief said. He said that all the children who survived their parents' attacks will be given counseling to help them recover.
The use of children in attacks in Indonesia is a "big development, unlike anything that's happened before in domestic terror," said Greg Barton, chair in global Islamic politics at Deakin University in Australia.
ISIS, from which the recent attackers reportedly took their cues, has always understood the power of using provocative violence, Barton said.
Families had been attracted to ISIS' previous calls for foreign Muslims to join the group in Iraq and Syria, as it was sold as a "utopian project." They were then radicalized "as a family unit," he said.
"We've seen IS (Islamic State) use its influence on kids -- its propaganda focuses on what they've called the 'cubs of the caliphate,' " he said.
"We've also seen children involved in executing political prisoners, and there's long been an appetite in Islamic State to use women and children."
The recent spate of bombings is the most deadly that Indonesia has experienced in more than a decade, and an unwelcome return to the post-9/11 climate that saw a flood of domestic terror activity, largely orchestrated and inspired by the al Qaeda-affiliated group Jemaah Islamiyah.
It claimed responsibility for 11 attacks between 2000 and 2010, including the deadly 2002 Bali bombings that left more than 200 people dead and hundreds injured, many of them tourists.
The clampdown was largely credited to the creation of a special anti-terror division called Densus 88, or D88.
"D88 quickly exceeded expectations," said Barton from Deakin University. "They (were) competent, created good counterintelligence through (questioning) and developed the capacity to detect splinter cells and apprehend militants."
However, the return of Indonesian fighters and their families from ISIS-controlled areas of Iraq and Syria presented a new challenge to the country's law enforcement and intelligence agencies.
"Once Islamic State emerged (in 2014) there was anxiety that this would create more movement in jihadi (circles)" and that Indonesians trained in ISIS territory would return home, Barton said.
While the families involved in the attacks over the last 48 hours were not known to have traveled to ISIS' so-called caliphate, the "returnee situation throws up potential other charismatic recruiters," Barton said.