Video report by ITV News Asia Correspondent Debi Edward
Sri Lankan officials failed to heed warnings from intelligence agencies about the threat of an attack by a local jihadist group that has been blamed for the bombings that killed almost 300 people, including eight Britons.
Prime Minister Ranil Wickremesinghe said he was not warned of information about planned attacks reportedly circulating amongst security chiefs earlier in April.
Minister for Telecommunications Harin Fernando tweeted what appeared to be information on "an alleged attack plan",claiming that "some intelligence officers were aware of this incidence.
"Therefore there was a delay in action... Serious action need to be taken as to why this warning was ignored."
The coordinated blasts that ripped through churches and luxury hotels were carried out by seven suicide bombers from a militant group named National Thowfeek Jamaath, Health Minister Rajitha Senaratne said.
No group has yet claimed responsibility for the attacks.
Up until now, National Thowfeek Jamaath (NTJ) had been a little-known terrorist group which had only been responsible for the defacing of Buddhist states in Sri Lanka.
However, it was widely known across the globe, and it was foreign agencies who tipped off Sri Lanka 10 days prior to Sunday's attack.
ITV News Security Editor Rohit Kachroo believes the level of scale and coordination NTJ had suggest it had help from groups outside of Sri Lanka.
As well as the devices that did go off, 87 detonators were discovered at Colombo Bus Station on Sunday night, while a 6ft pipe-bomb packed with 50kg of explosives was discovered near Colombo Airport has also been discovered.
Rohit Kachroo added that Sri Lanka has a well-developed and sophisticated counter-terrorism structure, suggesting that it could be part of an international network, possibly contracted out to a small, local group by a larger terrorist network such as Al-Qaeda or so-called Islamic State.
From the country, ITV News Asia Correspondent Debi Edward reported that police in Sri Lanka were seen to be very quick in the aftermath of the attack, but the fact that suicide bombers were able to walk into hotels and detonate bombs raises questions over security.
On Monday, a van parked near St Anthony's Shrine - one of the sites targeted on Sunday - exploded as police attempted to detonate three bombs attached to it.
No injuries were reported following the blast.
On Sunday, 290 people were killed in blasts at churches and upmarket hotels, and more than 500 people were also injured on one of the most important dates in the Christian calendar.
Sri Lanka's military have been granted sweeping war-time powers to detain and arrest suspects as of midnight, a curfew also came into affect from 8pm.
What do we know about the victims?
A father has paid tribute to his "wonderful wife" and "amazing children" who were killed in a bombing at the Shangri-la Hotel in Sri Lanka, one of the targets of Sunday's attacks.
Anita Nicholson, her 14-year-old son, Alex, and 11-year-old daughter, Annabel, were killed in the bombing as they ate breakfast in the hotel's restaurant, Ben Nicholson said.
The billionaire fashion tycoon behind online clothing retailer Asos lost three of his four children in the Sri Lanka terror attacks.
A spokesperson for Anders Holch Povlsen, Denmark’s wealthiest man, confirmed the deaths.
In a statement released on Monday, the Foreign Office confirmed that "at least eight British nationals were killed in yesterday’s horrific attacks in Sri Lanka.
"Our deepest condolences go to all those who lost loved ones and have been affected by this tragedy."
The majority of those killed in the attacks were Sri Lankans.
As well as the eight Britons and three Dutch children killed in the blasts, eight Indians, two Australians, two Chinese citizens, and people from the US, Japan and Portugal are said to be among the 39 foreigners killed.
What happened on Sunday?
Six near-simultaneous explosions took place shortly before 9am local time on Sunday morning - three at churches and three at luxury hotels.
As well as St Anthony's Shrine in Colombo, three luxury hotels - the Cinnamon Grand, Shangri-La and Kingsbury, were also targeted.
The explosion at the Shangri-La is thought to have taken place in the second-floor restaurant.
Other blasts were reported at St Sebastian's Church in Negombo, a majority Catholic town north of Colombo, and at Zion Church in the eastern town of Batticaloa.
A few hours later, two more blasts occurred just outside of Colombo, one of them at a guesthouse, where two people were killed, the other near an overpass.
Three police officers were killed during a search at a suspected safe house on the outskirts of Colombo when its occupants apparently detonated explosives to prevent arrest.
All of the bombers were Sri Lankan citizens, but authorities suspect foreign links, Health Minister Rajitha Senaratne said at a news conference.
Earlier, Ariyananda Welianga, a government forensic crime investigator, said an analysis of the attackers' body parts made clear that they were suicide bombers.
Have any arrests been made?
Some 24 people have been arrested in connection with the attacks.
Police have also seized a vehicle they believe was used to transport the suspects into Colombo, and have also found a safe house thought to have been used by the attackers.
Ms Gunasekera added that the investigations were moving "very swiftly" but warned against taking a "linear view" on the motive of the attacks.
"This cuts across the ethnic and religious dimensions ... it's very difficult to see who has been targeted," she told BBC Radio 4's Today programme.
"It appears as if the entirety of Sri Lanka has been targeted as well as the unity and coexistence that Sri Lankans have attempted so hard to safeguard over the years."
What have eyewitnesses said?
Lakmal, a 41-year-old businessman in Negombo was at St Sebastian's for Easter Mass with his family.
He said they all escaped the blast unscathed, but he remains haunted by images of bodies being taken from the sanctuary and tossed into a truck.
At the Shangri-La Hotel, a witness said "people were being dragged out" after the blast.
"There was blood everywhere," said Bhanuka Harischandra a 24-year-old from Colombo and founder of a tech marketing company.
He was heading to the hotel for a meeting when it was bombed.
"People didn't know what was going on. It was panic mode."
Footage shows the aftermath of the attacks
Julian Emmanuel was staying in the Cinnamon Grand hotel in Colombo with his family when he was woken up by the blast.
"We were woken up... by an explosion and we were then asked to evacuate the hotel through the stairs as the emergency alarm was going off and we were informed that there had been a blast," he told ITV News.
His family were ushered outside where they stayed for a couple of hours and witnessed injured guests and staff being taken to hospital and was informed some staff members had lost their lives.
"The children are a little traumatised from everything they saw this morning, especially seeing the injured people and some of the tears and some of the disappointed faces, so that was quite difficult for them," he added.
Is there a history of violence in Sri Lanka?
The attacks are the deadliest violence to hit Sri Lanka since the civil war in the country ended in 2009, which saw separatist Tamil Tigers and other rebel groups target the Central Bank, a shopping mall, a Buddhist temple and hotels popular with tourists.
Are there many Christians in Sri Lanka?
Only around 7.4% of Sri Lanka's population, or 1.5 million people, is Christian, the majority of whom are Roman Catholic.
According to the 2011 census, the dominant religion in the country is Theravada Buddhism of which 70.2% of the population identify as, followed by 12.6% Hindu and 9.7% Muslim.
What to do if you have been caught up in the attacks
The FCO is urging anyone directly affected by the attacks to call British Embassy in Colombo on +94 11 5390639, follow the phone message, and select the emergency option from where you will be connected to a member of consular staff.
The British Commissioner also told those in Sri Lanka (around 8,000 British citizens are currently believed to be in Sri Lanka) to contact loved ones at home.
What to do if you are concerned about a loved one in Sri Lanka
If you are in the UK and worried about British friends or family in Sri Lanka caught up in the incidents, the FCO can be contacted on 020 7008 1500 where you will be able to select an emergency option and then will be connected to a member of consular staff.
Mr Dauris also urged British citizens to try and make contact with loved ones.