FBI agents and U.S. military personnel have arrived in Sri Lanka to assist the investigation into a series of bombings at churches and luxury hotels on Easter Sunday that killed more than 350 and injured 500, the U.S. ambassador to Sri Lanka said.

Ambassador Alaina Teplitz said the U.S. had no prior knowledge of the attacks, despite local claims that foreign officials had been warned of an imminent assault.

The Islamic State claimed responsibility Tuesday for the attacks through its Amaq News Agency.

Sri Lanka's Defense Minister Ruwan Wijewardene told reporters Wednesday that most of the bombers were well-educated.

“Some of them have I think studied in various other countries, they hold degrees," including law degrees, "they’re quite well-educated people,” he said, according to the Guardian.

He said one of the bombers is believed to have studied in the United Kingdom and undertaken postgraduate studies in Australia before returning to Sri Lanka.

Its statement, two days after the deadly attacks, came after a senior government official said the suicide bombings were "in retaliation" for mass shootings at two mosques in New Zealand last month.

Neither the Islamic State, also known as ISIS, nor the Sri Lankan government offered any evidence. Monday, Sri Lankan Health Minister Rajitha Senaratne said seven members of the radical Muslim organization National Thowfeek Jamaath – a little-known domestic militant group that might have international ties – were behind the attacks.

ISIS, which has made unsupported claims of responsibility in previous strikes, released an image purportedly showing the leader of the attackers standing amid seven others whose faces are covered.

The identities of those in the image could not be independently verified, but CNN reported the man whose face can be seen is National Thowfeek Jamaath leader Zahran Hashim, also known as Mohammed Zahran.

The network said his name was revealed to Indian authorities by a suspected ISIS member who had been arrested, and Sri Lankan officials were warned about him. Hashim posted incendiary speeches online three years ago.

Prime Minister Ranil Wickremesinghe said investigators were determining the extent of the bombers’ foreign ties.

"This could not have been done just locally," Wickremesinghe said. "There had been training given and a coordination which we are not seeing earlier.''

The ISIS claim said, “The perpetrators of the attack that targeted nationals of the countries of the coalitions and Christians in Sri Lanka before yesterday are fighters from the Islamic State.”

The death toll rose from 321 to 359, police announced early Wednesday.

On Tuesday, Defense Minister Wijewardene told the country's parliament the Easter bombings were "in retaliation for the attack against Muslims in Christchurch," according to the Independent.

Fifty people were killed and dozens wounded in the attack in Christchurch on March 15 during Friday prayers. Brenton Tarrant, 28, a self-described white supremacist from Australia, was charged in the shootings.

Regarding retaliation, the office of New Zealand’s prime minister said it hadn't “seen any intelligence upon which such an assessment might be based," The Associated Press reported.

International intelligence agencies warned Sri Lankan officials of a possible attack April 4, and the Defense Ministry included the name of National Thowfeek Jamaath in a warning to the police chief on April 9, Senaratne said Monday.

Senaratne said Wickremesinghe and his Cabinet were unaware of the intelligence until after the attacks because of political dysfunction. Wickremesinghe and President Maithripala Sirisena have been feuding.

SRI LANKA BOMBINGS:What we know now

In a live address to the nation late Tuesday, Sirisena said he was kept in the dark on the intelligence about the planned attacks and vowed to “take stern action” against the officials who failed to share the information. He pledged “a complete restructuring” of the security forces.

Defense Secretary Hemasiri Fernando told the BBC that the intelligence "never indicated it was going to be an attack of this magnitude.''

"They were talking about isolated, one or two incidents. Not like this," he said.

President Sirisena said he gave the military wartime powers to arrest suspects. The military has not had such sweeping power since the country's 26-year civil war, which ended in 2009.

The government blocked access to online sites Facebook, WhatsApp and Instagram after the blasts, adding to the confusion and sense of chaos in parts of the country as residents and visitors sought assurance that the danger had passed. A nationwide state of emergency began at midnight Monday.

The bombers were all Sri Lankan, authorities said, but international influence is suspected.

''We, certainly the security apparatus, are of the view there are foreign links, and some of the evidence points to that,'' Wickremesinghe said. "So if the IS (Islamic State) claimed it, we will be following up on this claim."

Wickremesinghe warned there are more explosives and militants “out there.”

Police said 40 suspects had been arrested as of early Tuesday, including the driver of a van allegedly used by suicide bombers.

A total of nine bombings took place Sunday in the deadliest instance of violence in Sri Lanka since the civil war ended.

4 Americans killed

At least four Americans were among the dead, the U.S. State Department said.

The first American identified among the victims was Dieter Kowalski, 40, a Wisconsin native living in Colorado.

The U.S. State Department issued a travel advisory warning Americans to "exercise increased caution" because of terrorism concerns.

Sri Lankan authorities planned to brief foreign diplomats and receive assistance from the FBI and other foreign intelligence-gathering agencies.

Most of the dead were Sri Lankan, many of them members of the Christian minority. At least 30 foreign tourists were killed in the attacks. Victims from the United Kingdom, India, Australia, China, Japan, Spain and Portugal were confirmed.

The United Nations children’s agency said at least 45 minors were killed.

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Contributing: Jordyn Noennig, Milwaukee Journal Sentinel; Jane Onyanga-Omara, USA TODAY; The Associated Press