Two of his sons carried out attacks early Sunday morning, according to law enforcement. That afternoon as police entered the family residence, officials said, his daughter-in-law detonated explosives, killing herself and three officers.

The role played by the family in the devastating attacks that killed at least 250 people was one of several new details about the plot that emerged Wednesday, as Sri Lanka continued to bury its dead and investigators raced to understand whether the perpetrators had received help from abroad.

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The group that unleashed Sunday’s attacks on Sri Lanka included nine suicide bombers who detonated explosives in churches, hotels and inside the home belonging to the family, authorities said. The leader was a man named Zahran Hashim, who was believed to be in his 40s and expounded extremist views in online sermons. Authorities said the group’s leader carried out a suicide attack on Colombo’s Shangri-La Hotel.

The bombers were a radical splinter group that broke off from a local Islamist militant outfit called National Thowheed Jamaath, said Ruwan Wijewardene, Sri Lanka’s junior defense minister. Some of them had had previous run-ins with the law for minor offenses, he said.

The group included “quite well-educated people” from comfortable backgrounds, Wijewardene said. One perpetrator studied in Britain and pursued a master’s degree in Australia, he said.

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Authorities said they had identified eight of the nine bombers but declined to provide further details while the investigation was ongoing. On Wednesday, the nation’s Parliament passed regulations giving emergency powers to the police and military to detain suspects, and for the fourth day in a row, authorities imposed a nationwide nighttime curfew.

On Thursday, Sri Lankan health officials lowered the official death toll from 359 to at least 250 but said an exact figure was still not available.

In response to a growing furor about the government’s failure to act on earlier intelligence that warned of potential attacks, President Maithripala Sirisena ordered two top national security officials to resign. Pujith Jayasundara, the country’s police chief, and Hemasiri Fernando, the top civil servant at the Defense Ministry, stepped down.

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In Dematagoda, a quiet, prosperous neighborhood lined with spacious homes, sits the three-story residence belonging to a businessman known professionally as Y.M. Ibrahim and whom police refer to simply as Ibrahim. The house where he lives with his extended family takes up much of a block and was cordoned off with police tape Wednesday. Its facade is painted white and dotted with nine metal balconies.

Neighbors spoke fondly of Ibrahim, whom they described as a man from a modest background who rose to become a wealthy exporter of spices such as pepper and cinnamon. Ibrahim was one of at least 60 people arrested in the wake of the attacks, police spokesman Ruwan Gunasekara said.

How terror detonated with precision across Sri Lanka By Joe Fox, Terrence McCoy and Leslie Shapiro/Post, undefined, undefined and undefined A series of coordinated bombings — and the fear of more to come — has convulsed Sri Lanka since Sunday morning. This is how the attacks unfolded. AD AD

Piyal Siriwardena, 84, said Ibrahim would invite neighbors to an annual luncheon he held for hundreds of people in Colombo’s Pettah neighborhood. Politicians would attend, and Ibrahim once made a bid to join politics under the banner of a Marxist-leaning party.

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“He was a good, generous man,” Siriwardena said. “I’m shocked.”

Neighbors said the family largely kept to itself. They said the women of the house were rarely, if ever, seen outside, and none had had extended interactions with the sons. Several mentioned that the sons made a gesture during prayers they found unusual. The gesture is associated with Salafism, a fundamentalist brand of Islam.

Ibrahim, meanwhile, was a friendly man who talked with everyone at the small community center that serves as the local mosque. The violence allegedly wrought by the family of their neighbor stunned longtime residents such as Hussain Milhar, 53. “How could Ibrahim have children like this?” he asked.

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The Islamic State asserted responsibility for Sunday’s attacks, but Sri Lankan authorities said the group’s role remains unclear. Wijewardene, the junior defense minister, said there was a connection to the Islamic State “through ideology and maybe funding,” but the latter is still under investigation.

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In comments to reporters, the U.S. ambassador to Sri Lanka agreed that some degree of foreign assistance was likely. “If you look at the scale of the attacks, the level of coordination, the sophistication of them, it’s not implausible to think there are foreign linkages,” Alaina Teplitz said.

Wijewardene said Wednesday that investigators were exploring whether the bombers had traveled to neighboring countries such as India or Maldives, where they may have been trained or radicalized.

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He added that Sri Lanka’s intelligence agencies had assessed that revenge for last month’s attacks on mosques in Christ­church, New Zealand, was a factor. “What happened in New Zealand motivated these people to carry it out on Easter Sunday,” Wijewardene said. But he declined to provide further details on how that assessment was reached.

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Questions persisted about the failure by Sri Lankan authorities to stop the plot. A top police official had warned in an April 11 intelligence report that a radical group could be plotting suicide attacks on popular churches in Sri Lanka. The report named ­Thowheed Jamaath and its leader, Hashim.

Hilmy Ahamed, vice president of the Muslim Council of Sri Lanka, said in an interview that his organization had alerted intelligence officials three times that Hashim was stoking religious tensions and inciting hatred in online sermons, most recently in February.

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Police also revealed how efforts by pastors at Zion Church in the coastal city of Batticaloa probably saved lives. The bomber’s original target was St. Mary’s Cathedral, but he left when he realized that Mass was over, according to the senior police superintendent of Batticaloa, Nuwan Mendi.

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Instead, the bomber, carrying a backpack and another bag, headed for nearby Zion Church. He attempted to enter the sanctuary where some 500 people were gathered, but pastors were suspicious and stopped him. He detonated his explosives in the courtyard, killing at least 28 people, including children who had gathered to eat breakfast.

The country remained on edge Wednesday.

Authorities carried out three controlled explosions on two motorbikes and one package deemed suspicious. None of the items contained explosives. Wijewardene urged citizens to remain vigilant. “Within a couple of days, we can have total control of this situation,” he said.