At least 13 suspects were arrested on Monday in connection with Easter Sunday bombings in Sri Lanka that targeted churches and hotels, killing at least 290 people and leaving 500 others wounded, police said.

Authorities have not made public details on those held after Sunday's attacks, but a police source told AFP the 13 were detained at two locations in and around Colombo. The source said the 13 men are from the same group.

The Sri Lankan government believes a local Islamist extremist group called the National Thowheeth Jama'ath (NTJ) was behind the deadly suicide bomb attacks, government spokesman Rajitha Senaratne said.

According to Bloomberg, Hilmy Ahamed, vice president of the Muslim Council of Sri Lanka, had warned military intelligence officials about the group and its leaders about three years ago.

A government forensic crime analyst told The Associated Press that the six near-simultaneous attacks on three churches and three luxury hotels in and around Colombo on Easter Sunday were carried out by seven suicide bombers.

An analysis of the attackers' body parts made clear that they were suicide bombers, said Ariyananda Welianga, a forensic crime investigator. He said most attacks were by one bomber, with two at Colombo's Shangri-La Hotel.

At a glance:

• Four hotels ─ Cinnamon Grand, Shangri-La, Kingsbury and Tropical Inn ─ targeted in Colombo
• One church each targeted in Colombo (St Anthony's Shrine), Negombo (St Sebastian's Church) and Batticaloa (Zeon Church)
• 290 killed
• 36 foreigners among dead
• About 500 injured
• Curfew, 'temporary' social media ban imposed
• Attacks carried out by 7 suicide bombers

Police spokesman Ruwan Gunasekera declined to give a breakdown of those killed and wounded in each of the blasts. He added that police are investigating whether suicide bombers were involved in all of the blasts.

The government said the dead included three Indians, three Britons, two from Turkey and one Portuguese national. Two people holding both British and US passports were also among the fatalities.

"Additionally, while nine foreign nationals are reported missing, there are 25 unidentified bodies believed to be of foreigners," the foreign ministry said.

A government source said President Maithripala Sirisena, who was abroad when the attacks happened, has called a meeting of the National Security Council early today. Prime Minister Ranil Wickremesinghe will attend the meeting, the source said.

Meanwhile, Sri Lankan police investigating the bombings are examining reports that intelligence agencies had warnings of possible attacks, officials said on Monday.

Two government ministers have alluded to intelligence failures. Telecommunications Minister Harin Fernando tweeted, "Some intelligence officers were aware of this incidence. Therefore there was a delay in action. Serious action needs to be taken as to why this warning was ignored." He said his father had heard of the possibility of an attack as well and had warned him not to enter popular churches.

Mano Ganeshan, the minister for national integration, said his ministry's security officers had been warned by their division about the possibility that two suicide bombers would target politicians.

The police's Criminal Investigation Department, which is handling the investigation into the blasts, will look into those reports, Gunasekara said.

'International support' being probed

Government spokesman Senaratne, who is also a cabinet minister, said that the authorities were investigating whether the NTJ group had “international support”.

“We don't see that only a small organisation in this country can do all that,” he said.

“We are now investigating the international support for them, and their other links, how they produced the suicide bombers here, and how they produced bombs like this.”

Documents seen by AFP show Sri Lanka's police chief issued a warning on April 11, saying that a “foreign intelligence agency” had reported NTJ was planning attacks on churches and the Indian high commission.

Not much is known about the NTJ, a radical group that has been linked to the vandalising of Buddhist statues.

A police source told AFP that all 24 people in custody in connection with the attacks belong to an “extremist” group, but did not specify further.

Religiously motivated violence in SL

The scale of the bloodshed recalled the worst days of Sri Lanka's 26-year civil war, in which the Tamil Tigers, a rebel group from the ethnic Tamil minority, sought independence from the Buddhist-majority country. The Tamils are Hindu, Muslim and Christian.

According to a Dawn report, a common factor linking the church attacks was that they predominantly targeted Tamil Christians.

The St Anthony’s Catholic church in Kochchikde, located in a conclave near the Colombo port, was among the first to be bombed, when a mass in the Tamil language was going on.

The St Sebastian church in Negombo came under attack soon afterwards. Negombo is a popular tourist destination about 35km from Colombo.

Also coming under attack was the Christian Zion church in Eastern Batticaloa, a district which is populated primarily by Tamils and Muslims.

Only a small fraction of mainly Buddhist Sri Lanka is Catholic, but the religion is seen as a unifying force because it includes people from both the Tamil and majority Sinhalese ethnic groups.

Last year, there were 86 verified incidents of discrimination, threats and violence against Christians, according to the National Christian Evangelical Alliance of Sri Lanka (NCEASL), which represents more than 200 churches and other Christian organisations.

This year, the NCEASL recorded 26 such incidents, including one in which Buddhist monks allegedly attempted to disrupt a Sunday worship service, with the last one reported on March 25.

Out of Sri Lanka's total population of around 22 million, 70pc are Buddhist, 12.6pc Hindu, 9.7pc Muslim, and 7.6pc Christian, according to the country's 2012 census.

In its 2018 report on Sri Lanka's human rights, the US State Department noted that some Christian groups and churches reported they had been pressured to end worship activities after authorities classified them as "unauthorised gatherings".

Social media ban

The defence ministry said a temporary social media shutdown imposed yesterday would extend until the government concludes its investigation into the bomb blasts that rocked churches, luxury hotels and other sites.

However, NetBlocks observatory cautioned that such post-attack blackouts are often ineffective.

"What we've seen is that when social media is shut down, it creates a vacuum of information that's readily exploited by other parties," said Alp Toker, executive director of the London-based group. "It can add to the sense of fear and can cause panic."

The group said its monitoring of Sri Lankan internet connectivity found no disruptions to the fundamental infrastructure of the internet, meaning the blackout was directed at specific services. Some social media outlets, such as Twitter, appeared unaffected, but the blockage affected popular messaging services.

"That's going to be a problem for people trying to communicate with friends and family," Toker said.

Some internet users are circumventing the social media blocks by using a virtual private network, which masks the location of a computer, Toker said.

It isn't the first time Sri Lanka has blocked social media. The government imposed a weeklong ban in March 2018 because of concerns that WhatsApp and other platforms were being used to fan anti-Muslim violence in the country's central region.

An analysis by Sri Lankan researcher and author Yudhanjaya Wijeratne of thousands of Facebook posts made during last year's ban found that many Sri Lankans simply found ways around it.

Wijeratne has recommended narrower and more "technically challenging" approaches to curbing hate speech, such as better detection and strengthening local laws.

Facebook, which owns WhatsApp and Instagram, has struggled in recent years to combat the use of its platforms to incite violence and spread hate messages and political propaganda in countries including India, Myanmar and the United States.

The company said in a statement on Sunday that it has been working to support first responders and law enforcement in Sri Lanka and identify and remove content that violates company standards.

"We are aware of the government's statement regarding the temporary blocking of social media platforms," the company said. "People rely on our services to communicate with their loved ones and we are committed to maintaining our services and helping the community and the country during this tragic time."

Google didn't respond to a request for comment about the disruption to its YouTube service in Sri Lanka. Requests for comment made to messaging services Snap and Viber were not returned on Sunday.