Surabaya, Indonesia (CNN) A spate of deadly, ISIS-inspired bombings that rocked Indonesia's second-largest city in 24 hours were carried out by three families -- including their young children -- who targeted churches and the police, authorities said.

In the latest attack on Monday morning, a family of five rode two motorcycles to the front gate of Surabaya's police headquarters before detonating explosives, injuring 10 people.

On Tuesday, police identified the couple as Tri Murtiono and his wife Tri Ernawati, who carried out the attack accompanied by their sons, aged 18 and 14, and their 7-year-old daughter. CNN had previously reported that the girl was eight, per police statements.

She was riding as a passenger on one of the bikes and was thrown clear of the explosion, police spokesman Frans Barung Mangera said. A video of the scene showed her staggering through the rubble before a bystander picked her up and carried her to safety.

The bombing came one day after a family of six, including four children, detonated explosives at three churches , killing 12 people and injuring at least 40.

The father, identified by police as Dita Oepriarto, was said to have driven his wife Puji Kuswat and their two daughters, aged 9 and 12, to the Indonesian Christian Church. The trio went inside and detonated a bomb.

An old family photograph of Puji Kuswati and her children, who carried out suicide bombings in Surabaya on Sunday.

Oepriarto then drove the van to the Pentecostal Central Church, where, from inside the vehicle, he detonated another bomb, police said.

Around the same time the couple's two teenage sons, aged 16 and 18, drove motorcycles to the Santa Maria Catholic Church, where they also detonated bombs. All members of the family died in the attacks, which ISIS claimed responsibility for via its Amaq News Agency in what it called "a martyrdom operation."

Later Sunday, in what police also described as a terror incident, a mother and her 17-year-old daughter were killed in the Surabaya suburb of Sidoarjo when a bomb handled by the family's father detonated prematurely. Police found the father of the family in a house holding a detonator and shot him, police spokesman Barung Mangera said.

The family's 12-year-old son took his two younger sisters to the Bhayangkara Police Hospital, he added.

A damaged motorcycle is seen outside the Surabaya Centre Pentecostal Church in Surabaya on May 14.

Tito Karnavian, Indonesia's top-ranking police officer, told reporters Monday that police were working on the assumption that the attacks followed a directive from ISIS Central Command to avenge the imprisonment of the former leaders of Jamaah Ansharut Daulah (JAD) , an Indonesian jihadi group that supports ISIS.

Indonesia, the world's most populous Muslim country, has struggled in recent months with a rise in Islamist militancy, which has come as ISIS has been squeezed out of its heartland in Syria and Iraq.

Karnavian also told reporters Monday that none of the families involved in the attacks had recently traveled to Syria, but Oepriarto had close links with someone who had recently returned from Syria who may have inspired him to carry out the attacks.

"These attacks are the nightmare scenario that's been anticipated since Indonesians affiliated with ISIS have returned from the Middle East," said Greg Barton, Chair in Global Islamic Politics at Deakin University in Australia.

A government handout image shows a bomb blast at Surabaya Pantekosta (Pentecostal) Center Church on May 13, 2018 in Surabaya, Indonesia.

Families linked

In an exclusive interview with CNN, Oepriarto's father said his son was close friends with the man involved in the explosion in the suburb of Sidoarjo on Sunday evening.

"He's never spoken about it, but I know my son is close friends with Anton," Raden Doddy Oesodo said, referring to the man who died in the affordable housing complex in Sidoarjo after a bomb detonated prematurely.

"Anton was my son's buddy in high school. Anton is my son's junior within the JAD organization. My son, his wife and Anton were part of the same JAD membership," he said, adding that his son was introduced to JAD in high school.

Raden Doddy Oesodo, Dita Oepriarto's father, holds a family photo.

Oesodo described Oepriarto as "reclusive" and "private," but said he never spoke about martyrdom or traveling to Syria. "I've never heard him talk about jihad, but my son often disagrees with government policies."

Sitting in his home, Oesodo held a family photo of Oepriarto and his three other children and spoke about his love for his family.

"I love my grandchildren very much. They died because of their father's ideology," Oesodo said.

"I want to apologize to Surabaya residents who have become a victim due to my son's actions. I apologize from deep within my heart, especially to those who died because of him."

A candlelight vigil in the city of Medan on Indonesia's Sumatra island to support the victims and their relatives of a series attacks at churches in Surabaya.

'Opening salvo'

Formed in 2015, the group linked to the attacks, Jamaah Ansharut Daulah (JAD), is a jihadi group that supports ISIS, according to Jakarta officials.

Its leader, Aman Abdurrahman, was scheduled to appear at a court hearing last week, but it was postponed after a deadly riot broke out at the jail where Aman is being held in Depok, West Java, according to a report in the Jakarta Post.

Police now believe the riot at the Mobile Brigade Detention Center was initiated by a convict acting on the ISIS Central Command's instruction.

The militant group is part of a "second wave" of terrorist groups to be active in Indonesia, according to Hugo Brennan, senior Asia analyst at Verisk Maplecroft, a risk consultancy firm.

"The first wave was linked Al Qaeda from 2001 and were involved in the Bali attacks. Indonesian security forces dealt with them fairly effectively.

"Then in 2014 there was an uptick in violence as groups linked to ISIS became active, stoked by online propaganda, militants who traveled to the Middle East and fighting in the Philippines. The attacks yesterday are part of a pattern ... but (are among) the more sophisticated, and as it involved children, heinous."

Officers block a road following an attack at the local police headquarters in Surabaya, East Java, Indonesia.

Barton at Deakin University said the Surabaya attacks could be the "opening salvo of a new, more sophisticated campaign," and added that the timing -- just before Ramadan, which begins May 15 -- "bears the hallmarks of ISIS," which has used the holy month in the past to launch high profile attacks.

"This is the first Ramadan since ISIS-linked Indonesians have returned" from Syria, he said.

Indonesia has a long history with terrorist groups, particularly the al-Qaeda affiliated group Jemaah Islamiyah, which 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.

According to Wawan Purwanto, the information and communication direct of Indonesia's National Intelligence Agency, around 930 Indonesians have traveled to Syria and Iraq to fight alongside ISIS since 2013.

Of those, around 400 had returned to Indonesia while 100 are known to have died in battle. Exact numbers are uncertain because immigration officials in Iraq and Syria don't have a record of them entering those countries, Wawan said.

Surabaya's mayor Tri Risma Harini being evacuated away as Indonesian police stand guard outside the Surabaya police station following another explosion.

The United Nations Secretary-General condemned the three terrorist attacks on the Surabaya churches. Through his spokesman, Stephane Dujarric, Secreatry General Antonio Guterres said that he was "appalled at reports that children were used to participate in the attacks," and offered his condolences to the families of the victims.

"(The Secretary General) reiterates the support of the United Nations to the Government and people of Indonesia in their efforts to fight and prevent terrorism and violent extremism, including through the promotion of pluralism, moderation and tolerance," the statement said.

More than 82% of Indonesia's roughly 261 million people follow Islam. Almost 10% of the population is Christian.

Correction: The name of the father in the church suicide bombings has been updated following further information from police. His name is Dita Oepriarto.