Three attacks, including one by a suicide bomber disguised as a churchgoer, targeted three churches in Indonesia's second largest city of Surabaya, killing at least eight people and wounding dozens.
The series of attacks on Sunday were carried out by the Islamic state-inspired group Jemaah Ansharut Daulah (JAD), a spokesman for the country's intelligence agency said.
Wawan Purwanto, communication director at the agency, also told Metro TV that the attacks were likely to be linked to a deadly prison hostage incident at a jail near Jakarta involving Islamist militants last week.
Asked who he thought were the brains behind the attacks, Purwano said: "Still the old group, JAD, who has planned this for sometime."
"They had planned to attack police targets on May 11 but because the police were prepared, they picked alternative targets," he told Metro TV.
The first attack struck during Sunday Mass at the Santa Maria Catholic Church, killing four people, including the suspected bomber, police spokesman Frans Barung Mangera told reporters at the scene. He said police officers were among a total of 38 wounded.
It was followed by a second explosion minutes later at the Christian Church of Diponegoro that killed two people. Another two died in a third attack at the city's Pantekosta Church, Mangera said.
The latest attacks in predominantly Muslim Indonesia came days after police ended a riot and hostage-taking at a detention center near Jakarta that left six officers and an inmate dead. The Islamic State group has claimed responsibility.
Indonesia has carried out a sustained crackdown on militants since bombings by al-Qaida-affiliated radicals in Bali in 2002 killed 202 people. In recent years the country has faced a new threat as the rise of the Islamic State group in the Middle East invigorated local militant networks.