Members of a single family carried out the suicide bombings that killed at least 11 people and wounded dozens at three churches in Indonesia's second-biggest city on Sunday, police said.
The attacks during Sunday services, claimed by ISIS, were the deadliest in years, as the world's biggest Muslim-majority country grapples with homegrown militancy and rising intolerance towards religious minorities.
The bombers — a mother and father, two daughters aged 9 and 12, and two sons aged 16 and 18 - were linked to Jamaah Ansharut Daulah (JAD), a local extremist network which supports ISIS, said national police chief Tito Karnavian.
The mother, identified as Puji Kuswati, and her two daughters were wearing niqab and had bombs strapped to their waists as they entered the grounds of the Kristen Indonesia Diponegoro Church and blew themselves up, he said.
The father, JAD cell leader Dita Priyanto, drove a bomb-laden car into the Surabaya Centre Pentecostal Church while his sons rode motorcycles into Santa Maria church, where they detonated explosives they were carrying, Mr Karnavian said.
"All were suicide attacks but the types of bombs are different," he said.
"This is related to JAD — Jamaah Ansharut Daulah."
East Java police spokesman Frans Barung Mangera confirmed the deaths of 11 people with 41 injured in the co-ordinated attacks at around 7.30am.
Images showed a vehicle engulfed in flames and plumes of thick black smoke as a body lay outside the gate of Santa Maria Catholic church, with motorcycles toppled over amid the mangled debris.
In addition to the suicide blast police experts defused two unexploded bombs at the Surabaya Centre Pentecostal Church.
Indonesian President Joko Widodo slammed the attacks, telling reporters: "We must unite against terrorism."
"The state will not tolerate this act of cowardice."
Also on Sunday, police said four suspected JAD members had been killed in a shoot-out during raids linked to a deadly prison riot last week.
Five members of Indonesia's elite anti-terrorism squad and a prisoner were killed in clashes that saw Islamist inmates take a guard hostage at a high-security jail on the outskirts of Jakarta. ISIS claimed responsibility for the incident.The JAD, led by jailed radical Aman Abdurrahman, has been linked to several deadly incidents, including a 2016 gun and suicide attack in the capital Jakarta that left four attackers and four civilians dead. It was the first assault claimed by ISIS in South-East Asia.
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Nearly 90 per cent of Indonesia's 260 million people are Muslim, but there are significant numbers of Christians, Hindus and Buddhists.
Concerns about sectarian intolerance have been on the rise, with churches targeted in the past.
Police shot and wounded an ISIS-inspired radical who attacked a church congregation outside Indonesia's cultural capital Yogyakarta with a sword during a Sunday mass in February. Four people were injured.
In 2000 bombs disguised as Christmas gifts delivered to churches and clergymen killed 19 people on Christmas Eve and injured scores more across the country.
The archipelago nation of some 17,000 islands has long struggled with Islamic militancy, including the 2002 Bali bombings that killed 202 people — mostly foreign tourists — in the country's worst-ever terror attack.
Sunday's bombings had the highest death toll since nine people were killed in 2009 attacks on two luxury hotels in Jakarta.
Security forces have arrested hundreds of militants during a sustained crackdown in recent years that smashed some networks, and most recent attacks have been low-level and targeted domestic security forces.
But the co-ordinated nature of Sunday's bombings suggested a higher level of planning, analysts said.
"Recent attacks have been far less 'professional'," said Sidney Jones, an expert on South-East Asian terrorism and director of the Jakarta-based Institute for Policy Analysis of Conflict.
The emergence of ISIS has provided a rallying point for radicals, sparking fears that home-grown extremist outfits could get a new lease of life.