(CNN) The calm of Easter Sunday was shattered by gruesome bombings that killed at least 250 people in Sri Lankan churches and hotels.

Though it's not clear who's behind the eight explosions that forced the country of 21 million people to go on lockdown, they are "certainly acts of terror," said Manisha Gunasekera, high commissioner of Sri Lanka to the UK.

"Serious action need to be taken as to why this warning was ignored," Sri Lanka's Minister of Telecommunications, Harin Fernando, tweeted along with a photo of the memo. The document, titled "Information of an alleged plan attack," is dated April 11 and signed by the Deputy Inspector General of Police, Priyalal Dissanayake.

The explosions blew out the tiled roofs of churches and hotel windows, killing worshippers and hotel guests in the process.

Images and footage showed bloodied pews, broken glass, and plumes of smoke.

"You can see pieces of flesh thrown all over the walls and on the sanctuary and even outside of the church," Father Edmond Tillekeratne, social communications director for the Archdiocese of Colombo, told CNN from St. Sebastian's Church, one of the explosion sites.

He estimated that more than a thousand people had come to the church for Easter Sunday "because it is a special day." Many came from villages afar, he said.

Photos: In pictures: Sri Lanka on alert as mourners bury blast victims A priest conducts a burial ceremony for victims of the Easter Sunday bombings in Negombo, Sri Lanka, on Wednesday, April 24. Hide Caption 1 of 39 Photos: In pictures: Sri Lanka on alert as mourners bury blast victims A woman who lost her husband and two children during the bombing at St. Sebastian's Church in Negombo yells toward the grave site during a mass burial on Wednesday. Hide Caption 2 of 39 Photos: In pictures: Sri Lanka on alert as mourners bury blast victims Military personnel talk with a police officer near the site where a package, believed to be a bomb, was detonated in a controlled explosion after being discovered in a nearby restaurant in Negombo on Wednesday, April 24. Hide Caption 3 of 39 Photos: In pictures: Sri Lanka on alert as mourners bury blast victims Members of a security task force search for explosives ahead of victims' mass burials in Negombo on Wednesday. Hide Caption 4 of 39 Photos: In pictures: Sri Lanka on alert as mourners bury blast victims On Tuesday, April 23, Harshani Sriyani weeps over the body of her daughter who was killed in the Easter Sunday bombings in Colombo. Hide Caption 5 of 39 Photos: In pictures: Sri Lanka on alert as mourners bury blast victims Funeral ribbons hang across a road leading to St. Anthony's Shrine on April 23. Hide Caption 6 of 39 Photos: In pictures: Sri Lanka on alert as mourners bury blast victims A man is supported as he follows a coffin during a mass funeral for bombing victims at St. Sebastian Church in Negombo, Sri Lanka, on Tuesday, April 23. Hide Caption 7 of 39 Photos: In pictures: Sri Lanka on alert as mourners bury blast victims A soldier stands guard next to members of the clergy during a mass funeral in Negombo on Tuesday. Hide Caption 8 of 39 Photos: In pictures: Sri Lanka on alert as mourners bury blast victims Coffins are carried to a grave during the mass funeral at St. Sebastian Church on April 23. Hide Caption 9 of 39 Photos: In pictures: Sri Lanka on alert as mourners bury blast victims A woman carrying an infant runs for safety after police found a suspicious vehicle parked in Colombo, Sri Lanka, on Monday, April 22, a day after several coordinated bombings across the country killed hundreds. Hide Caption 10 of 39 Photos: In pictures: Sri Lanka on alert as mourners bury blast victims Sri Lankan security forces stand at the site near St. Anthony's shrine in Colombo after they performed a controlled detonation of a suspicious van on Monday. Hide Caption 11 of 39 Photos: In pictures: Sri Lanka on alert as mourners bury blast victims Smoke rises from the area where a van exploded on Monday near St. Anthony's shrine in Colombo. Hide Caption 12 of 39 Photos: In pictures: Sri Lanka on alert as mourners bury blast victims Security personnel stand guard outside St. Anthony's Shrine on Monday, a day after the church was hit in a series of bomb blasts targeting churches and luxury hotels in Sri Lanka. Hide Caption 13 of 39 Photos: In pictures: Sri Lanka on alert as mourners bury blast victims Relatives weep near the coffin of 12-year-old victim Sneha Savindi, who was killed in the Easter Sunday bombing at St. Sebastian's Church in Negombo, Sri Lanka. Hide Caption 14 of 39 Photos: In pictures: Sri Lanka on alert as mourners bury blast victims Police and forensics officials inspect the interior of St. Sebastian's Church in Negombo on Monday. Hide Caption 15 of 39 Photos: In pictures: Sri Lanka on alert as mourners bury blast victims Shoes and belongings of victims are collected as evidence at St. Sebastian's Church in Negombo on April 22. Hide Caption 16 of 39 Photos: In pictures: Sri Lanka on alert as mourners bury blast victims A man mourns after viewing the body of a dead relative killed in the bomb explosion at St. Anthony's Church in Colombo on Sunday, April 21. Hide Caption 17 of 39 Photos: In pictures: Sri Lanka on alert as mourners bury blast victims Blood stains are seen on a statue of Jesus Christ after a bomb blast inside a church in Negombo, on Sunday. Hide Caption 18 of 39 Photos: In pictures: Sri Lanka on alert as mourners bury blast victims A woman is in tears after a deadly bomb blast at St. Anthony's Shrine in Sri Lanka's capital, Colombo, on Sunday. Hide Caption 19 of 39 Photos: In pictures: Sri Lanka on alert as mourners bury blast victims Security personnel and investigators sift through debris outside Zion Church in Batticaloa. Hide Caption 20 of 39 Photos: In pictures: Sri Lanka on alert as mourners bury blast victims A man holds up a blood-stained photograph as people wait to identify the bodies of their loved ones in front of St. Anthony's Church. Hide Caption 21 of 39 Photos: In pictures: Sri Lanka on alert as mourners bury blast victims Relatives of a victim of the attacks react at the police mortuary in Colombo. Hide Caption 22 of 39 Photos: In pictures: Sri Lanka on alert as mourners bury blast victims Sri Lankan hospital workers and soldiers stand at the door to a morgue following a blast in a church in Batticaloa. Hide Caption 23 of 39 Photos: In pictures: Sri Lanka on alert as mourners bury blast victims A Sri Lankan Air Force helicopter flies over a house suspected to be a hideout of militants following a shootout in Colombo. Hide Caption 24 of 39 Photos: In pictures: Sri Lanka on alert as mourners bury blast victims A view of damage near the Dehiwala zoo in Colombo. Hide Caption 25 of 39 Photos: In pictures: Sri Lanka on alert as mourners bury blast victims Relatives of bombing victims gather at a mortuary in Colombo. Hide Caption 26 of 39 Photos: In pictures: Sri Lanka on alert as mourners bury blast victims Colombo's Kingsbury Hotel was targeted by a blast. Hide Caption 27 of 39 Photos: In pictures: Sri Lanka on alert as mourners bury blast victims Relatives of a blast victim grieve outside a morgue in Colombo. Hide Caption 28 of 39 Photos: In pictures: Sri Lanka on alert as mourners bury blast victims Sri Lankan Special Task Force personnel gesture outside a house during a raid following an explosion at a property in the Orugodawatta district of Colombo. Hide Caption 29 of 39 Photos: In pictures: Sri Lanka on alert as mourners bury blast victims The interior of St. Sebastian's Church in Negombo, north of Colombo, shows damage from a bomb blast. Hide Caption 30 of 39 Photos: In pictures: Sri Lanka on alert as mourners bury blast victims Sri Lankan military members stand guard outside St. Anthony's Church following an explosion. Hide Caption 31 of 39 Photos: In pictures: Sri Lanka on alert as mourners bury blast victims Crime scene investigators inspect the scene of an explosion at the Shangri-La hotel in Colombo. Hide Caption 32 of 39 Photos: In pictures: Sri Lanka on alert as mourners bury blast victims Sri Lankan police officers clear the road as an ambulance drives through carrying injured from bomb blasts in Colombo. Hide Caption 33 of 39 Photos: In pictures: Sri Lanka on alert as mourners bury blast victims A bombing victim's relative weeps outside a hospital in Batticaloa, eastern Sri Lanka. Hide Caption 34 of 39 Photos: In pictures: Sri Lanka on alert as mourners bury blast victims Priests view blast debris outside St. Anthony's Shrine. Hide Caption 35 of 39 Photos: In pictures: Sri Lanka on alert as mourners bury blast victims Sri Lankan security personnel keep watch outside St. Anthony's Shrine. Hide Caption 36 of 39 Photos: In pictures: Sri Lanka on alert as mourners bury blast victims Security forces inspect the inside of St. Anthony's Shrine after Sunday's attack. Hide Caption 37 of 39 Photos: In pictures: Sri Lanka on alert as mourners bury blast victims Sri Lankan military officers stand guard in front of St. Anthony's Shrine. Hide Caption 38 of 39 Photos: In pictures: Sri Lanka on alert as mourners bury blast victims A relative of a victim of an explosion at St. Anthony's Shrine reacts outside the police mortuary in Colombo. Hide Caption 39 of 39

"This is an attack against the whole of Sri Lanka because Sri Lanka is (a) multi-ethnic, multi-religious and multi-cultural country, and the whole country comes together in celebration of Easter Sunday," Gunasekera said.

The violence punctured a decade of relative peace in the country following the end of its civil war in 2009 -- where attacks were common during the 25-year struggle.

Since then, Sri Lanka has turned itself into a popular tourist destination, winning the title of best place in the world to visit in 2019 from travel guide publisher Lonely Planet

Four days after the coordinated attacks, the Sri Lankan Health Ministry revised the death toll, saying 253 people had died. That toll is significantly lower than the 359 initially reported to CNN by a Colombo police spokesman this week. The health ministry cited the condition of remains and the difficulty in identifying them for the discrepancy.

How the attacks unfolded

St. Sebastian's Church in Negombo was severely damaged in the attacks.

The first wave of attacks struck during busy Easter services at churches in the heart of the country's minority Christian community -- in the cities of Colombo, Negombo and Batticaloa.

More blasts ripped through three luxury hotels in the capital city of Colombo: the Shangri La, Cinnamon Grand and Kingsbury.

The Shangri-La in Colombo said the hotel's Table One cafe was attacked just after 9 a.m local time. The hotel is popular with foreign tourists and the country's business community.

Another blast rocked a hotel in front of the Dehiwala Zoo in Dehiwala-Mount Lavinia.

The final blast struck a private house in Mahawila Gardens, in Dematagoda, during a raid in connection with the attacks, officials said. Three police officers were killed.

"When crime division officials started questioning the people in the house, two explosions occurred," police spokesman Ruwan Gunasekara said. "One sub inspector and two constables of police have been killed."

Sri Lankan Prime Minister Ranil Wickremesinghe "expressed his gratitude" to the officers in a statement, according to state news agency Derena TV.

Wickremesinghe reiterated that "stern action will be taken against those responsible," the news agency reported.

In all, the eight sites of explosions Sunday included:

St. Anthony's Shrine, Colombo

St. Sebastian's Church, Negombo

Zion Church, Batticaloa

Cinnamon Grand, Colombo

Shangri-La Hotel, Colombo

The Kingsbury Hotel, Colombo

Near Dehiwala Zoo in Dehiwala-Mount Lavinia

A house in Mahawila Gardens, Dematagoda

"There is also information that these have been suicide bombings carried out," Manisha Gunasekera said.

There was no immediate claim of responsibility by any terror group. But seven people were arrested following the attacks, said Harsha de Silva, Sri Lanka's minister of economic reforms and public distribution.

The victims came from around the world

A Sri Lankan officer inspects a blast site at the Shangri-La hotel in Colombo.

Of the hundreds of people killed, about 30 were foreigners, de Silva tweeted

At least 20 foreigners were among those killed in the capital city of Colombo, hospital director general Anil Jasinghe said.

The foreigners killed include at least three Indians, two Turkish nationals and two Chinese citizens, Sri Lanka's Foreign Ministry said in a statement.

Five British citizens, including two dual US-UK nationals, were also killed in the bombings, the statement said.

Social media is cut off

The Sri Lankan government blocked social media sites such as Facebook and Instagram while the investigation got under way.

Facebook issued a statement Sunday offering its condolences to the those "affected by this horrendous act."

"Teams from across Facebook have been working to support first responders and law enforcement as well as to identify and remove content which violates our standards," the company said.

"We are aware of the government's statement regarding the temporary blocking of social media platforms. 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."

Authorities order a curfew

Police in Sri Lanka imposed an island-wide curfew from Sunday evening until Monday morning. In Colombo, the streets were eerily quiet, with hardly any cars on the road and no signs of bustling city life.

Four guards carrying AK-47 rifles stood by a locked gate of a hotel not far from the Shangri-La, and bomb-sniffing dogs checked bags.

Earlier, the country's authorities convened an emergency meeting involving the heads of the army, air force and navy, according to de Silva, Sri Lanka's economic reforms minister.

Sec Defence and I am at Kochchikade church. Also was at ShangriLa n Kingsbury. PM is on his way from Bentota. Emergency meeting called in a few minutes. Rescue operations underway. Please stay calm and indoors. Many casualties including foreigners. — Harsha de Silva (@HarshadeSilvaMP) April 21, 2019

"Horrible scenes, I saw many body parts strewn all over," de Silva said after visiting the Kochchikade church and Shangri-La Hotel in Colombo. "We took multiple casualties to hospital. Hopefully saved many lives."

De Silva said rescue operations were underway, adding that emergency crews were operating in "full force."

Christians were the apparent targets

Sri Lanka's minority Christian community appeared to be the main target of Sunday's attack. Christianity is a minority religion in Sri Lanka, accounting for less than 10% of the total population of 21.4 million.

According to census data, 70.2% of Sri Lankans identify as Buddhist, 12% Hindu, 9.7% Muslim, and 7.4% Christian. It is estimated that 82% of Sri Lankan Christians are Roman Catholic.

Sunday's attacks risk upsetting the country's fragile post-war peace. Tensions between the majority Sinhalese and the Tamil minority led to a 25-year civil war between the Tamil Tigers, classified by the United States and others as a terrorist organization, and government forces.

More than 70,000 people died in the fighting , which ended when Sri Lankan forces defeated the Tamil Tigers in 2009.

In recent years, the country has witnessed a surge in ultra-nationalist Buddhism led by the Bodu Bala Sena, the country's most powerful Buddhist organization, which has pledged to defend the religion.

Update: This story has been updated to reflect the death toll has been revised by the Sri Lankan Health Ministry.