Latest developments:

The death toll in the Easter Sunday suicide attacks on churches and hotels has risen to 321.

ISIS has claimed responsibility, but Sri Lankan officials have blamed two local Islamic extremist groups for the bombings, possible with outside help.

A senior Sri Lankan officials says the preliminary investigation shows the attack was "retaliation" for the March 15 massacre of Muslims at mosques in New Zealand.

Video has emerged showing one of the bombers walking calmly into a church on Easter Sunday with a large backpack.

At least 40 people have been arrested.

Tuesday is a national day of mourning in Sri Lanka, which has been placed under strict emergency laws enabling police to make arrests without a court order.

ISIS claimed responsibility on Tuesday for the Easter Sunday suicide bombings in Sri Lanka, as an official in the country said the preliminary investigation into the attacks showed it was "retaliation" for the massacre of Muslims at mosques in Christchurch, New Zealand. The coordinated bombings on Sunday, targeting churches and hotels in and around the Sri Lankan capital, killed more than 320 people.

"The preliminary investigations have revealed that what happened in Sri Lanka was in retaliation for the attack against Muslims in Christchurch," state minister of defence Ruwan Wijewardene told parliament. It was unclear what Wijewardene based his assessment on, and later on Tuesday Prime Minister Ranil Wickremesinghe was more guarded, saying it was "possible" there was a connection between the two attacks.

Fifty people were killed in shooting attacks on two mosques in the New Zealand city of Christchurch on March 15. An Australian white supremacist is accused of carrying out the attack on his own, and streaming his acts on Facebook.

Get Breaking News Delivered to Your Inbox

The death toll in the Sri Lanka bombings had risen to 321 by Tuesday, which had been declared a national day of mourning after the devastating attacks. At least 45 of the dead were children, according to the United Nations children's agency.

The now-landless Islamic State of Iraq and Syria (ISIS), issued a claim of responsibility for the bombings on Tuesday with a short statement from the group's propaganda wing. ISIS' self-styled "news agency" Amaq circulated a statement claiming, "the perpetrators of the attack... were Islamic State fighters."

As it has done after claiming previous attacks around the world, the group followed up later in the day with another release containing more information, including a photo showing the group of militants purportedly behind the suicide bombings. It showed eight men, all but one of them masked, and most carrying knives.

A photo released by ISIS' propaganda wing Amaq on April 23, 2019, shows eight men purported by the terror group to be the suicide bombers who carried out the coordinated attacks on three mosques and four hotels in Sri Lanka on Easter Sunday.

The statement issued by Amaq on Tuesday identified the purported bombers only by pseudonyms.

Prime Minister Wickremesinghe said Tuesday that investigators were making good progress in identifying the culprits, and he added that some of of the bombers had travelled abroad and returned home before carrying out the attacks. At least 40 suspects had been taken into custody for questioning as of Tuesday, all of them Sri Lankan nationals.

CBS News correspondent Elizabeth Palmer reported that while the Sri Lankan officials have provided little detail on their investigation into the attacks, new video emerged overnight showing one of the bombers walking calmly into St. Sebastian's Church in Negombo, north of Colombo. Dozens were killed in the blast that followed.

Sri Lankans woke up on Tuesday with their country under an official state of emergency -- a declaration that gives law enforcement agencies the power to arrest and detain people without charge or court order.

Sri Lankan officials on Monday pinned the blame for the attack on a domestic Islamic extremist group, the National Thowfeek Jamaath (NTJ), which they said had likely acted with the help of an "international network." On Tuesday, Wijewardene said another Sri Lankan extremist group, Jammiyathul Millathu Ibrahim (JMI) also took part in the attack "along with" the NTJ.

While there has been no official implication of ISIS in the Sri Lanka attacks thus far, CBS News law enforcement analyst Paul Viollis told CBSN on Monday that the way it was carried out appeared right out of the al Qaeda or ISIS "playbook."

An intelligence alert was sent to top Sri Lankan law enforcement officers on April 11, warning that suicide bombers from NTJ were planning to attack "prominent churches" in the country.

Two government ministers alluded separately on Monday to possible intelligence failures before the attack.

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."

The FBI is assisting Sri Lankan authorities in their investigation into the attacks, but the exact nature of that assistance remained unclear. CBS News correspondent Jeff Pegues notes that in the case of foreign attacks, the FBI often offers its evidence collection and analysis capabilities, which are among the best in the world. Sources confirmed to Pegues that FBI agents are on the ground in Sri Lanka assisting with the investigation.