At least 290 people are now known to have died in a coordinated attack on churches and hotels on Easter Sunday in Sri Lanka, in what officials have called a "brand-new type of terrorism."

Police have arrested 24 people in connection with the suicide bombs, which injured at least 500 people, in the worst violence the South Asian island has seen since its bloody civil war ended 10 years ago.

Authorities were facing accusations that they had failed to act on a warning received ten days before the atrocity that an Islamist group was preparing an attack.

There were fears of more devices: A ninth improvised explosive device (IED) was defused near the capital's Bandaranaike International Airport on Sunday evening, according to an Air Force spokesman.

Most of the dead and injured were Sri Lankan. Of the foreign nationals who died, five were British citizens, two of whom held dual US-UK nationality; as well as three Indians, two Australians, two Chinese cousins, one person from the Netherlands, two Turkish citizens and one Portuguese national. The blasts appears to have targeted tourism hotspots, as well as churches, in an effort to gain maximum global attention.

No group has claimed responsibility, but Sri Lankan Defense Minister Ruwan Wijewardene said the “terrorist incident” was carried out by followers of “religious extremism.”

 On Sunday evening, a leaked memo revealed that police were warned of a potential attack by an Islamist group known as the Nations Thawahid Jaman (NTJ), ten days before the Easter Sunday bombings. It is unclear whether the details contained in the warning matched the atrocity that eventually took place.

Sri Lanka's Prime Minister, Ranil Wickremesinghe, said the intelligence was not shared with him or other ministers. Sajith Premadasa, minister of housing construction and cultural affairs, said officers were guilty of “negligence and incompetence.”

There were doubts that the NTJ, a little-known group which has previously defaced Buddhist statues, would have had the capacity to carry out such a sophisticated and coordinated attack alone. Transnational Islamists are known to operate in places like Pakistan, Malaysia and the Philippines.

But Dhruva Jaishankar, a fellow in Foreign Policy Studies at Brookings India, said that little is known about Islamic radicalism in Sri Lanka and that it was “premature” to speculate on which organizations might have been involved.

A social media blackout was enforced in the aftermath of the attacks as authorities attempted to contain the violence and establish who carried out the attacks. A curfew was imposed from 8 p.m. to 4 a.m. local time.

Christianity is a minority religion in Sri Lanka, accounting for less than 10% of the total population of 21.4 million. According to census data, 70.2% of Sri Lankans identify as Buddhist, 12% Hindu, 9.7% Muslim, and 7.4% Christian. It is estimated that 82% of Sri Lankan Christians are Roman Catholic.

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