COLOMBO, Sri Lanka — As authorities investigated the worst attacks to strike this island nation since its civil war ended a decade ago, the focus Monday turned to an obscure local Islamist group, possible ties to extremists abroad — and the question of why the government did not do more to prevent the carnage.

Officials on Monday blamed the highly coordinated suicide bombings of churches and hotels Sunday on National Thowheed Jamaath, a little-known group based in eastern Sri Lanka. A U.S. official said the group has links to the Islamic State, but their significance is unclear. Investigators here and abroad were working to determine whether the group received help from overseas.

“We do not believe these attacks were carried out by a group of people who were confined to this country,” said Rajitha Senaratne, a cabinet minister. “There was an international network without which these attacks could not have succeeded.”

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Ten days before the Easter bombings, a senior police official warned that Thowheed Jamaath was “planning suicide attacks” against targets that could include well-attended Catholic churches. In a three-page report sent to senior security officials on April 11, the police official identified the group and at least five alleged members by name.

Overnight on Monday, the death toll was revised to 311, according to state minister for defense Ruwan Wijewardene; more than 500 people were wounded. Among the dead were four Americans, including Kieran Shafritz de Zoysa, a fifth-grader at Washington’s Sidwell Friends School who was spending the year in Sri Lanka and was to return to the school next year.

Sri Lanka was tense and fearful Monday as mourners began to bury their dead and the government invoked emergency measures to track down the perpetrators.

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Authorities made more arrests and detonated an explosive device found near one of the churches that was attacked. They instituted a dusk-to-dawn curfew for a second day, continued to block access to Facebook and other social media sites and watched for signs of reprisal attacks.

President Maithripala Sirisena sought international assistance with the investigation.

President Trump called Sri Lankan Prime Minister Ranil Wickremesinghe on Monday to express condolences and pledge U.S. support. The FBI was sending agents, offering laboratory expertise in testing bomb evidence and scouring databases for information that could shed additional light on the plotters.

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The U.S. official, who spoke on the condition of anonymity to discuss a still unfolding matter, said Thowheed Jamaath has links to the Islamic State, but it was not clear how significant they are. Whether they go beyond merely expressing support via social media, for example, had not been determined. Nor had any operational coordination between the Islamic State and the Sri Lankan group been established.

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Sirisena invoked arrest powers for the military not used here since 2011, and authorities had detained more than 20 people, including several Indian and Pakistani citizens.

Some were arrested at a large home in Colombo belonging to the family of Thowheed Jamaath’s alleged leader, according to a police official who spoke on the condition of anonymity.

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That leader is believed to have died in a suicide attack on the Shangri-La Hotel in Colombo, according to investigators who were not authorized to speak to the media.

A Sri Lankan security official said there could be additional ­explosives or potential suicide bombers that authorities have not yet located.

“Right now, they are searching everywhere for possible bombs and people involved,” said the official, who spoke on the condition of anonymity to discuss the ongoing investigation.

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The official described Thowheed Jamaath as a shell for the Islamic State that has been active in the eastern region of Kattankudy, home to a large Muslim population. The group’s leadership is believed to be based there.

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Michael Leiter, who served as director of the National Counterterrorism Center in the George W. Bush and Obama administrations, said the group “wasn’t on anyone’s radar.”

He said the attacks probably had an international nexus, given that they did not target only Sri Lankans.

“It wouldn’t surprise me either if there were at least a couple of people who had traveled to Syria,” Leiter said. “There was never a large Sri Lankan population there, but it only takes one or two to return and inspire a local group to align itself ideologically and tactically with a global violent jihadist organization.”

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Nicholas Rasmussen, who ran the National Counterterrorism Center in the Obama and Trump administrations, said the absence of a clear claim of responsibility from an established international terrorist organization suggests it might be too soon to say whether the Sri Lankan bombers had outside assistance.

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“But it wouldn’t take much — a connection between Sri Lankan foreign fighters in Syria with like-minded people back home — in order to create such a connection,” Rasmussen said. He said the simultaneous attacks and the high death toll suggested a degree of sophistication in bombmaking and organization “characteristic of an established group.”

Michael Morell, a former deputy director of the CIA, said a number of young Sri Lankan Muslims did leave the country to fight with the Islamic State.

While authorities still did not know the background of the attackers, he said, “it is possible that at least some of them were among the tens of thousands who traveled to Iraq and Syria to fight for ISIS from 2014 to 2017.”

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Morrell, who now hosts the Intelligence Matters podcast, called the attacks “an important reminder that jihadist terrorism still remains a significant threat.”

The SITE Intelligence Group, which tracks extremist activity online, said Monday that an unidentified Islamic State supporter distributed photos of three alleged “commandos” involved in the Sri Lanka attacks.

The photos were posted in pro-Islamic State chat rooms, SITE said, and the men, pictured holding weapons in front of Islamic State banners, were described as “among the commando brothers in Sri Lanka.”

Islamic State supporters have portrayed the attacks as revenge for strikes on mosques and Muslims, SITE said.

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Criticism grew Monday of the government’s failure to act on ­prior intelligence about Thowheed Jamaath and its plans.

Two government officials provided The Washington Post with the report issued by police deputy inspector general Priyalal Dassanayake on April 11.

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Dassanayake identified the group and at least five alleged members by name, including the alleged leader, and warned that they were planning suicide attacks that could target churches and the Indian High Commission.

Dassanayake wrote that authorities were monitoring their social media posts and possessed other intelligence, including information on their whereabouts.

 The leader’s social media posts did not reveal “clear evidence” of an intent to attack churches, Dassanayake wrote, but had called since 2016 for “nonbelievers” to be killed.

“Confidential investigations are continuing,” he concluded.

 A spokesman for Sri Lanka’s police declined to comment on the report’s contents.

In the days after April 11, the report circulated in parts of Sri Lanka’s security establishment but was not raised before the cabinet.

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Mujibur Rahman, a member of Sri Lanka’s Parliament, said his security team was aware of the report before the attacks. He said the report was based on information provided by Indian intelligence agencies.

 The report has become a weapon in the bitter rivalry between Sri Lanka’s president and prime minister, who represent separate parties.

Sirisena, the president, also serves as the minister of defense. Allies of Wickremesinghe, the prime minister, say the handling of the report was a major national security blunder.

Others said the Sri Lankan government had done too little. Hilmy Ahamed, vice president of the Muslim Council of Sri Lanka, said he gave intelligence officials details on extremists in the community, including suspects in the current investigation, three years ago.

“I personally have gone and handed over all the documents,” Ahamed said. “They have sat on it. That’s the tragedy.”

A predominantly Buddhist nation, Sri Lanka is also home to significant numbers of Hindus, Muslims and Christians. Authorities remained on guard against rising tensions among communities.

In Negombo, where an attack on a church killed more than 100 people Sunday, a group of Muslim refugees from Pakistan was threatened and intimidated by residents. The refugees were taken under police protection.

The country remained on edge Monday as police continued to turn up explosive devices. An official with the Sri Lankan air force said an explosive was defused close to Bandaranaike International Airport, the city’s main airport, on Sunday night, and police conducted a controlled explosion Monday of a van parked near St. Anthony’s Shrine in Colombo, where dozens were killed Sunday.

Suicide bombers struck churches in the cities of Colombo, Negombo and Batticaloa on Easter Sunday, the holiest day of the Christian calendar. They also targeted three luxury hotels in Colombo: the Shangri-La, the Cinnamon Grand and the Kingsbury.

Six of the blasts occurred between 8:45 and 9:30 a.m. Sunday. A seventh occurred at a banquet hall about 2 p.m., with an eighth at the house police raided about 2:45 p.m.

The Shangri-La Hotel was attacked as guests gathered in the restaurant for breakfast. Investigators who spoke on the condition of anonymity because they were not authorized to talk to the media said two suspects had checked into a room at the hotel earlier in the morning and given local addresses.

Officers were killed by an explosion when they raided a house at one of those addresses, the Sri Lankan security official said.

The Rev. Jude Chrysantha said he rushed to Colombo’s St. Anthony’s Shrine on Sunday upon hearing of the blast there. He saw scores of dead bodies, including children.

He said the eruption of violence in a country that was growing accustomed to peace was unbearable.

“Even during the war, no one did harm to religious places,” Chrysantha said. “People just wanted to celebrate Easter in a joyful way. Now it’s a funeral for the whole country.”

Shibani Mahtani in Hong Kong, Rukshana Rizwie and Devana Senanayake in Colombo, Niha Masih in New Delhi, Chico Harlan in Rome and Souad Mekhennet, Devlin Barrett and Julie Tate in Washington contributed to this report.