Rajitha Senaratne, the Sri Lankan health minister, blamed the group at a news conference in Colombo, the capital, adding: “There was an international network without which these attacks could not have succeeded.”
The government announced that it was asking other countries for help in uncovering international links, and that it was assuming emergency powers in order to investigate the attacks. A dusk-to-dawn curfew was imposed for the second consecutive night.
No one has claimed responsibility for the attacks, which targeted Roman Catholic churches holding Easter services and high-end hotels favored by foreign tourists. On Monday, officials said the death toll had risen to at least 290, with about 500 others wounded. The Sri Lankan authorities have so far arrested two dozen suspects, but declined to identify them.
Prime Minister Ranil Wickremesinghe said on Sunday that he and other top government officials had not been informed of the threat, and that “we must look into why adequate precautions were not taken.”
A forensic analysis by the Sri Lankan government of human remains found at three churches and three hotels determined that seven suicide bombers had carried out the attacks. Most sites were attacked by lone bombers, but two targeted the Shangri-La Hotel in Colombo. Bombings at a guesthouse and the suspects’ safe house remain under investigation.
In interviews, counterterrorism experts said that such an extensively planned and coordinated attack would almost certainly have required considerable financing and expertise from a more experienced group overseas.
Sri Lanka, an island nation in the Indian Ocean, was ravaged by decades of civil war that ended in 2009, but it has little history of militant Islamist violence. The suicide bombings that were pioneered there starting in the 1980s were carried out by guerrillas from the country’s Tamil ethnic minority who were mainly Hindu, not Muslims.
Anne Speckhard, the director of the International Center for the Study of Violent Extremism, contrasted the attacks by Tamil guerrillas with those attributed to National Thowheeth Jama’ath. Unlike the bombings on Sunday, she said, those during the civil war were part of a nationalist or ethnic separatist movement, and generally did not have religious targets.
“These attacks appear to be quite different,” she said, “and look as if they came right out of the ISIS, Al Qaeda, global militant jihadist playbook, as these are attacks fomenting religious hatred by attacking multiple churches on a high religious holiday.”
National Thowheeth Jama’ath is a small but violent group of young Muslims that started at least three years ago in eastern Sri Lanka, far from the country’s more cosmopolitan western and southern coasts. Until this month, the group was generally perceived as anti-Buddhist, counterterrorism experts said.
Radical Islamist groups like Al Qaeda also have experience in organizing and carrying out simultaneous suicide attacks — most notably those in the United States on Sept. 11, 2001.
Local ties with such groups may have been strengthened in recent years by Sri Lankan Muslims who traveled to fight in wars in Syria and Iraq, said Sameer Patil, a national security fellow at Gateway House, a foreign policy research group in Mumbai, India. With the Islamic State having recently lost its last patch of territory in Syria, he said, the group’s foreign fighters are now more likely to return home to Sri Lanka and other countries.
“It was just a matter of time before that would hit them on their own soil,” Mr. Patil said.
Scott Stewart, the vice president of tactical analysis at Stratfor, a geopolitical consulting firm based in Austin, Tex., noted that the attackers were unusually successful for a group with no track record of large-scale assaults.
The initial evidence showed that all seven suicide vests detonated. That is certain to worry law enforcement agencies. Initial efforts by small, homegrown extremist groups are usually marked by some degree of failure. Some of the bombs fail to detonate entirely. Others explode early or late, and still others cause smaller blasts than their builders intended.
Whoever designed the suicide vests used in the Easter Sunday blasts showed considerable expertise, he said, and photographs indicate that the bombmaker had access to a lot of military-grade high explosives.
But Joshua A. Geltzer, who was senior director for counterterrorism in the Obama administration and is now the executive director of Georgetown University’s Institute for Constitutional Advocacy and Protection, said that it was now much easier for relatively unknown to groups to be successful on their first try.
“The notion that a smaller, regional terrorist group could pull off an attack of this magnitude really doesn’t stun me, in this age of digitally shared terrorist strategies and tactics,” Mr. Geltzer said. “There is so, so much instruction and guidance available on the open internet these days — not to mention whatever is circulating on encrypted chat groups, widely available in terrorist circles if not totally public.”
Ms. Speckhard said the aim of National Thowheeth Jama’ath was to spread the global jihadist movement to Sri Lanka and to create hatred, fear and divisions in society.
“It is not about a separatist movement,” she said. “It is about religion and punishing.”
Sectarian divisions are ripe for exploitation in Sri Lanka, whose ethnic Sinhalese majority is mostly Buddhist. Sri Lankan Muslims, who make up about one-tenth of the population and mostly speak Tamil, have a long history of conflict with the country’s Buddhists and Hindus, said Brahma Chellaney, a professor of strategic studies at the Center for Policy Research in New Delhi.
The conflict stems partly from statues and other human portrayals of Buddhist and Hindu deities, which some Muslims perceive as idolatrous.
In recent years, Buddhist extremist groups have sprung up among the Sinhalese, who make up nearly three-quarters of the Sri Lankan population. The groups seek to protect statues of Buddha from desecration and make Buddhism more central to Sri Lankan life, but they have also fomented violence. Last year, the Sri Lankan government declared a nationwide state of emergency after mob attacks against Muslims in the central district of Kandy.
Mr. Chellaney noted that there was also a large group called Thowheeth Jama’ath in the southeastern Indian state of Tamil Nadu, which lies across a strait from Sri Lanka and has a large Tamil population. Smaller chapters with the same name appear to have been set up in Sri Lankan communities in other countries, he said, often funded by groups in the Persian Gulf and subscribing to Wahhabism, an austere form of Islam that has its roots there. Mr. Chellaney described them as “sister organizations” but said the links among them were unclear.
Counterterrorism experts said that in Sri Lanka, National Thowheeth Jama’ath appeared to consist almost entirely of young people, especially recent graduates of Islamic schools. The group appears to have little hierarchy or organizational structure, and no older leaders.
The presence of mostly young people with deep roots in the community but no strong, mature leaders would make it similar to local groups elsewhere in the Muslim world with which the Islamic State has tried to form affiliations.