COLOMBO, Sri Lanka — Sri Lanka on Tuesday described the devastating string of bombings on Easter that killed 321 people as a response to the attack on two mosques in New Zealand last month.

Three hotels and three churches were attacked by suicide bombers on Sunday belonging to the radical Islamist group National Thowheed Jamaath in an operation that authorities appear to have had advance warning about.

"Investigations have revealed that the attacks were carried out by Islamic extremists in retaliation to the mosque attacks in Christchurch, New Zealand," State Minister of Defense Ruwan Wijewardene told parliament. On March 15, a white supremacist killed 50 Muslims in two mosques.

He did not offer any evidence for the connection and also acknowledged there were security lapses that allowed the attacks to occur, which he ascribed to rivalries between the president and the prime minister.

"Don't take this as a joke: As long as the division between the president and the prime minister exists, you can't solve this problem — my security division knew about the advance notice [of the attack], I did not."

This Sunday, April 21, 2019, file photo shows the inside of St. Sebastian's Church damaged in blast in Negombo, north of Colombo, Sri Lanka. The deadly Easter attacks in Sri Lanka are a bloody echo of decades past in the South Asian island nation, when militants inspired by attacks in the Lebanese civil war helped develop the suicide bomb vest.
Chamila Karunarathne / AP
This Sunday, April 21, 2019, file photo shows the inside of St. Sebastian's Church damaged in blast in Negombo, north of Colombo, Sri Lanka. The deadly Easter attacks in Sri Lanka are a bloody echo of decades past in the South Asian island nation, when militants inspired by attacks in the Lebanese civil war helped develop the suicide bomb vest.

Leaked copies of a report by intelligence officials earlier this month warned of plans by the National Thowheed Jamaath group to attack churches. Health Minister Rajitha Senaratne has called for the resignation of the top police official for not taking any action.

By Tuesday morning, 40 people had been arrested, including three being held by the Terrorism Investigation Deparment, said police spokesman Ruwan Gunasekara.

Police have been given emergency powers to detain and question suspects without a court order. Such powers were used extensively during Sri Lanka's civil war but have not been implemented since 2011.

It was also announced that schools and universities would be closed at least until Monday, and Masses at churches canceled until further notice. The country has been on edge with three bomb scares, including one at the U.S. Embassy, taking place in the last 24 hours.

Police have been instructed to look out for five bikes, a cab and a van suspected of carrying more explosives.

Funerals, meanwhile, were being held for the victims.

"Endless crying" is how Malini Vijaysingha, 60, described the hours since the attack as she paid her respects outside one of the bombed churches. She blamed the bombings on the Islamic State. "The whole world should destroy IS," she said. The links between the perpetrators and the Islamic State remain unclear.

The United States pledged support for the investigation, dispatching FBI agents to help, including by offering laboratory experience to test bomb evidence. At least four U.S. citizens are among the dead, and "several" Americans were seriously injured, the State Department said Monday. Sri Lankan Tourism Minister John Amaratunga said 38 foreigners were killed and 28 wounded.

Investigators will be looking into how the local Islamist group, whose name roughly translates to National Monotheism Organization, was able to carry out such a planned, coordinated and deadly attack and whether they had overseas help, as officials suggested Monday. President Maithripala Sirisena asked for international assistance in determining any foreign links.

As news of the supposed advance notice about the attacks spread, mourners responded with rising anger mixed with grief at funerals and other gatherings in Christian communities.

"This is the government's fault. They are incompetent. They knew and they did nothing," said one man who was weeping Monday outside a funeral in Negombo. He did not give his name, but turned away and joined others entering a house where the coffin of a woman lay on a cloth-covered table, surrounded by silent mourners.

Two officials provided The Washington Post with the three-page intelligence report that the health minister alluded to, in which a senior police official warned of potential suicide attacks by the same Islamist extremist group.

The authenticity of those documents were verified by Sri Lanka's state minister for defense, Ruwan Wijewardene. The report also identified several members by name, including the group's alleged leader.

Mujibur Rahman, a member of Sri Lanka's Parliament who was briefed on the report, said it was based on information from Indian intelligence agencies.

Authorities said the main attacks — on churches and hotels — were carried out by seven suicide bombers.

An investigator at the scene of a suicide bombing at St. Sebastian Church in Negombo, Sri Lanka, Monday, April 22, 2019. Easter Sunday bombings of churches, luxury hotels and other sites was Sri Lanka's deadliest violence since a devastating civil war in the South Asian island nation ended a decade ago.
Gemunu Amarasinghe / AP
An investigator at the scene of a suicide bombing at St. Sebastian Church in Negombo, Sri Lanka, Monday, April 22, 2019. Easter Sunday bombings of churches, luxury hotels and other sites was Sri Lanka's deadliest violence since a devastating civil war in the South Asian island nation ended a decade ago.

A Sri Lankan security official characterized Thowheed Jamaath as a shell for the Islamic State and said it has been active in Kattankudy, an area in the eastern part of the country and home to one of its largest Muslim populations. The group's leadership is believed to be based there, the official said.

The official said there could be additional explosives or potential suicide bombers.

"Right now, they are searching everywhere for possible bombs and people involved," the official said, speaking on the condition of anonymity to discuss the investigation.

Secretary of State Mike Pompeo blamed "Islamic radical terror" for the attacks. He also spoke Monday morning with Wickremesinghe and pledged "all possible assistance" to Sri Lanka.

"This is America's fight, too," Pompeo said at a news conference. Although the Islamic State's "caliphate" has been destroyed with the collapse of the group's last strongholds in Syria, "radical Islamist terror remains a threat," he said. "We have to remain active and vigilant, and it's going to require attention."

Thowheed Jamaath "wasn't on anyone's radar," said Michael Leiter, who served as director of the National Counterterrorism Center in the George W. Bush and Obama administrations. He said the attack probably had an international nexus, given that not only Sri Lankans were targeted.

"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."

But the absence of any 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, said Nicholas Rasmussen, a former senior director for counterterrorism on the National Security Council who also ran the National Counterterrorism Center in the Obama and Trump administrations.

"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 added that the high death toll and simultaneous attacks suggested a degree of sophistication in bombmaking and organization, which are "characteristic of an established group."

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, and the men, pictured holding weapons in front of Islamic State banners, were described as "among the commando brothers in Sri Lanka," SITE said.

The group reported Sunday that Islamic State supporters were portraying the attacks as revenge for strikes on mosques and Muslims.

The highly coordinated attacks left the island nation reeling, a crushing blow after almost a decade of peace since the end of its civil war.

In that time, tourism in Sri Lanka had been steadily growing, the country transformed by the apparent end of instability, bloodshed and frequent suicide bombings over the 26-year war.

A three-minute silence was observed countrywide at 8:30 a.m. Tuesday.

Tensions remain high across the island nation. The U.S. Embassy in Colombo said in a tweet that a bomb disposal unit had verified a suspicious package near the embassy building was not an explosive device.

The incident was the third such bomb scare over the past 24 hours.

Mahtani reported from Hong Kong. The Washington Post’s Rukshana Rizwie, Harshana Thushara Silva and Devana Senanayake in Colombo, Niha Masih in New Delhi, Shane Harris, Souad Mekhennet, Devlin Barrett and Julie Tate in Washington contributed to this article.