Members of one family were behind a wave of blasts targeting three churches in Indonesia’s second city of Surabaya, police say.
At least 11 people were killed and dozens others injured in the attack.
A mother blew herself and two children up at one church, while the father and three sons targeted two others, police chief Tito Karnavian said.
Sunday’s bombings, which the Islamic State group has claimed, are the deadliest in Indonesia since 2005.
Earlier in the day, Wawan Purwanto, of Indonesia’s intelligence agency, said an IS-inspired group, Jemaah Ansharut Daulah (JAD), was suspected to be behind the attacks.
What do we know about Sunday’s attack?
The first explosion took place between services at the Santa Maria Catholic Church around 07:30 local time (00:30 GMT). Inspector General Machfud Arifin told CNN Indonesia that a motorbike was used in the attack.
According to Reuters, the second bombing targeted the cark park of a Pentecostal church. Images of the scene showed a number of burnt motorcycles.
Eyewitness reports say that the third attack was carried out by a veiled woman who entered a church with two children.
TV pictures showed debris scattered around the entrance of one church.
Officials reportedly foiled attacks against other churches.
Visiting the scene of one of the bombings, President Joko Widodo described the attacks as “barbaric”, adding that he had ordered police to “look into and break up networks of perpetrators”.
Also on Sunday, police said they killed four suspected members of JAD in Cianjur, West Java, and arrested two others.
By BBC Indonesia Editor Rebecca Henschke
The blasts were co-ordinated to hit those coming to morning services and were over within the space of just 10 minutes.
In recent years women have become increasingly active in terrorist cells in Indonesia but this would be the first time children have been used.
Indonesia had been widely praised for its sustained anti-terrorism crackdown following the 2002 Bali bombings, a seemingly successful combination of arrests and killings alongside a de-radicalisation program that focused on changing minds and providing alternative incomes for released terrorists.
But the rise of IS overseas has invigorated the loosely constituted jihadi networks.
There has also been rising intolerance in recent years in this once tolerant pluralist majority-Muslim nation, making minorities groups here increasingly uncomfortable.
How does this compare to previous attacks?
This appears to be the worst attack linked to IS in Indonesia.
The group claimed its first attack in the country in January 2016, when four civilians were killed in a series of explosions and shootings in the capital Jakarta.
It has also claimed other attacks, such as violence earlier this month in a high-security prison near the capital Jakarta, in which five members of security forces were killed.
But the country’s deadliest jihadist attacks predate IS.
In 2002, over 200 people were killed in two bombings carried out by al-Qaeda-linked militants outside a bar and nightclub on the resort island of Bali.
In May 2005, bomb blasts killed 22 on the island of Sulawesi. Less than six months later, a suicide bombing in Bali killed 20.