Sri Lankan authorities are investigating what role international terrorist networks may have played in a string of suicide bombings on Easter Sunday after the Islamic State group claimed responsibility for the gruesome attack.

The death toll from the blasts, which targeted churches and hotels across the island nation, has risen to 359 people, police said, including 39 foreigners. At least 45 children were among the victims, according to UNICEF.

And on Wednesday, Sri Lankan President Maithripala Sirisena said he had asked the country's defense secretary and national police chief to resign, according to a report from The Associated Press. Officials have been heavily criticized over the government's handling of recent warnings from international intelligence agencies that an attack on Sri Lankan sites was likely.

Sri Lankan government officials have blamed the Islamist group National Thowheed Jamath (NTJ) for the attacks. The bombers were characterized as a "splinter group" of the organization with connections abroad, said State Minister of Defense Ruwan Wijewardene at a press conference Wednesday.

ISIS claimed responsibility for the attack on Tuesday and released a video of men pledging allegiance to ISIS chief Abu Bakr al-Baghdadi — although the group produced no evidence to link it to the attacks.

The validity of the video and identities of the men have not been confirmed, according to Flashpoint Intelligence, a global security firm and NBC News consultant.

The video claimed to identify Zahran Hashim, a Sri Lankan extremist cleric and preacher for NTJ, as the alleged leader behind the attacks, Flashpoint said.

Coffins are carried during a mass funeral at St. Sebastian Church on April 23, 2019 in Negombo, Sri Lanka. Carl Court / Getty Images

NTJ is a little-known group previously linked to the defacing of Buddhist statues in the country. It has not been linked to prior deaths.

Wijewardene called on the government in a tweet on Tuesday to ban the NTJ and list it as a terrorist organization.

Islamist extremist groups, specifically ISIS and al-Qaeda, have struggled to develop a foothold in South Asia beyond Afghanistan, said Champa Patel, the head of the Asia-Pacific program at Chatham House, a London-based think tank.

Anti-Muslim bigotry fed by Buddhist nationalists has swept the country recently, but there is no history of Islamist militancy. Muslims make up only 9.7 percent of the country's 22 million people who are mostly Buddhist. Christians account for another 7.6 percent of people according to the country's 2012 census.

Given the complexity of the near-simultaneous bombings at multiple locations, terrorism and security experts believe the assailants had support from external, more experienced operatives. Recent media reports suggest a man arrested in India ahead of the attacks has claimed to have trained the alleged attackers, according to Flashpoint.

Wijewardene, the deputy defense minister, said the link between the NTJ and ISIS is being investigated. He described the suspects involved as mostly well-educated from middle- or upper-middle-class backgrounds.

At least one of the bombers had studied in the United Kingdom and completed a post-graduate degree in Australia before resettling in Sri Lanka, Wijewardene said.

He referred to a "lapse of security arrangements" for the failure to take precautions despite warning from international intelligence agencies that an attack was likely.

The country remains on high alert. Authorities carried out a controlled explosion of a scooter on Wednesday morning following concerns there was something suspicious inside the seat.

The country's airports have increased security, Wijewardene said. International intelligence agencies including the FBI and Interpol are also providing support to Sri Lankan authorities.

Dozens of people have been arrested, all of whom are Sri Lankan, but officials did not confirm whether any of the suspects held dual citizenships.

Devastated families began holding mass burials for the dead on Tuesday. The bodies of 17 foreigners have been returned to their families to date, Wijewardene said.

Wasanthi, a member of Berlington family weeps over the gave of Bevon, who was killed in the Easter Sunday bombings in Colombo, Sri Lanka. Eranga Jayawardena / AP

A fifth-grade student from Washington, D.C., Kieran Shafritz de Zoysa, was among those killed. His father, Alexander Arrow, told NBC News that his son was having breakfast at the Cinnamon Grand Hotel in Colombo when a suicide bomber entered and detonated his backpack.

Not all the victims have yet been identified by authorities. Sri Lankan Americans told NBC News that they have struggled to contact their loved ones in the country since the government banned social media access while the investigation is underway.

Sehan Jayathunga, 24, said people have been buying more international calling cards from his father's grocery store on Staten Island after Viber Messenger and WhatsApp has failed to work.

“People just came in and they were devastated and worried,” he said.

CORRECTION (April 24, 2019, 09,26 AM ET): An earlier version of this article misstated that the Sri Lankan defense secretary, Hemasiri Fernando, had resigned. The president, Maithripala Sirisena, has asked him to resign within 24 hours; he has not resigned yet.