A series of bombings at churches and luxury hotels in Sri Lanka that killed more than 320 people on Easter Sunday were conducted "in retaliation" for mass shootings at two mosques in New Zealand last month, a senior government official said.

"The preliminary investigations have revealed that what happened in Sri Lanka was in retaliation for the attack against Muslims in Christchurch," Ruwan Wijewardene, Sri Lanka's defense minister, told the country's parliament, according to the Independent.

He said the death toll now stands at 321 dead and 500 injured.

Fifty people were killed and dozens wounded in the attack in Christchurch on March 15 during Friday prayers. A 28-year-old Australian man, Brenton Tarrant - a self-described white supremacist - has been charged over the shootings.

Seven members of the radical Muslim group National Thowfeek Jamaath - a domestic militant group that might have international ties - were behind the Sri Lanka attacks, Health Minister Rajitha Senaratne said Monday.

At least four Americans are among the dead, the U.S. State Department said. The department gave no details about the identities of the victims, citing privacy concerns, and said it provided support to Americans affected by the blasts.

Sri Lankan authorities on Tuesday planned to brief foreign diplomats and receive assistance from the FBI and other foreign intelligence-gathering agencies after officials disclosed Monday that warnings had been received weeks ago of the possibility of an attack by the radical Muslim group blamed for the bloodshed.

International intelligence agencies began warning the country's officials April 4, and on April 9, the Defense Ministry included the group's name in a warning to the police chief, Senaratne said.

Seranatne said Prime Minister Ranil 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.

More:Sri Lanka bombings: What we know

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 late Monday that he gave the military wartime powers to arrest suspects. The military has not had such sweeping power since the country's civil war, which ended in 2009. A curfew was to begin at 8 p.m.

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 was scheduled to begin at midnight Monday.

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

"We do not believe these attacks were carried out by a group of people who were confined to this country,” Senaratne said, according to Reuters. “There was an international network without which these attacks could not have succeeded."

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.

President Donald Trump called Wickremesinghe on Monday morning to express his condolences, White House spokesperson Hogan Gidley said. "The near simultaneous attacks on Sri Lankan churches and hotels constitute one of the deadliest terrorist events since the Sept. 11, 2001, attacks on the United States," Gidley said.

The FBI said it's providing assistance in the investigation.

Officials found 87 bomb detonators in Colombo, the country's capital. Twelve detonators were at the main bus depot and 75 in a garbage dump.

A van parked near St. Anthony’s Church in Colombo, which was hit in the bombings Sunday, exploded Monday. Police tried to defuse three bombs in it but detonated the vehicle instead. There were no injuries.

A forensic crime analyst told The Associated Press that an analysis of the attackers’ body parts collected from the scenes showed that the attacks were suicide bombings.

Two people were involved in the attack at the Shangri-La hotel. One bomber each attacked the Cinnamon Grand and Kingsbury hotels and St. Anthony’s Shrine in Colombo, St. Sebastian’s church in the city of Negombo and Zion Church in the city of Batticaloa.

Hours later, two bombings took place at a guesthouse and near an overpass on the outskirts of Colombo. Suspects detonated explosives at a safe house on the outskirts of Colombo, killing three police officers.

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.

The first American identified among the victims was Dieter Kowalski, 40, a Wisconsin native living in Colorado. His employer, education company Pearson, confirmed Kowalski's death Monday. CEO John Fallon said in a message on LinkedIn that Kowalski had just arrived at his hotel when he was killed in the explosion.

Officials at Sidwell Friends, a Washington-area private school where former President Barack Obama's daughter Sasha is a senior, confirmed the death of one of its students in an email to parents. The Washington Post reported the fifth-grade boy, Kieran Shafritz de Zoysa, had been on leave for the past year in Sri Lanka.

The Bestseller clothing chain confirmed Danish media reports that three of the children of its owner, business tycoon Anders Holch Povlsen, were killed in the attacks. Swedish newspaper Aftonbladet said Holch Povlsen, 46, and his wife, Anne, were vacationing in Sri Lanka with their children.

Victims from the United Kingdom, India, Australia, China, Japan, Spain and Portugal were confirmed.

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

The U.N. Security Council, the organization's most powerful body, condemned the bombings as “heinous and cowardly terrorist attacks” and called for the perpetrators to be held accountable.

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