Senior officials deliberately withheld intelligence about possible attacks, says leader of parliament.
Minister says the initial investigation shows Easter bombings were retaliation against New Zealand mosque attack.
Death toll rises to 359, with at least 500 injured.
Ministers say local Islamist movement, the National Thowheeth Jama'ath (NTJ), was behind the bombings.
So-called Islamic State claiming responsibility for the bombings, but did not provide any evidence to support this.
Rift between Sri Lanka's president and prime minister under scrutiny, with questions over how the government handled a warning of an attack.
There was an international network without which these attacks could not have happened, he added.
Bomb explodes as police try to defuse it outside St Anthony's Church.
Sri Lankan President Maithripala Sirisena has asked the police chief and defence secretary to quit following the Easter Sunday suicide bombs, Reuters reported on Wednesday, citing two sources close to the president.
They declined to be named due to the sensitivity of the matter amid accusations within the government of intelligence failures ahead of the attacks.
Senior officials deliberately withheld intelligence about possible attacks on Sri Lanka, where a wave of suicide bombings on Easter Sunday killed at least 359 people, according to the leader of the country's parliament.
"Some top intelligence officials hid the intelligence information purposefully. Information was there, but the top brass security officials did not take appropriate actions," Lakshman Kiriella, who is also minister of public enterprise, told parliament.
Indian intelligence containing information on possible suicide attacks on churches, hotels and politicians were received on April 4, according to the minister.
A Security Council meeting was chaired by President Maithripala Sirisena on April 7, but the information was not shared more widely.
"Somebody is controlling these top intelligence officials," the minister said. "The Security Council is doing politics. We need to investigate into this [sic]," he said.
The United States had no prior knowledge of the Easter Sunday suicide bombing attacks on churches and hotels in Sri Lanka, but now believes there is ongoing terrorism plotting in the country, Washington’s ambassador to Colombo said on Wednesday.
“We had no prior knowledge of these attacks,” Ambassador Alaina Teplitz told reporters in Colombo. “We believe there are ongoing terrorist plots. Terrorists can strike without warning. Typical venues are large gatherings, public spaces.”
President Maithripala Sirisena announced in an address to the nation on Tuesday that he will “restructure the police and security forces in the coming weeks” and change “the heads of defence establishments within the next 24 hours".
“The security officials who got the intelligence report from a foreign nation did not share it with me. I have decided to take stern action against these officials."
The announcement came after health Minister Rajitha Senaratne told reporters on Monday that authorities had been warned two weeks ago that an attack was imminent.
“On April 9, the chief of national intelligence wrote a letter and in this letter, many of the names of the members of the terrorist organisation were written down,” Senaratne said.
“The prime minister was not informed by [sic] these letters and revelations," he added.
Wickremesinghe, meanwhile, blamed a “ breakdown in communication” between his office and the President for the information not being shared and added that the difference of opinion between the two men had now been “thrashed out.”
The President is responsible for national security and intelligence, but tensions between the two men have been high since Sirinesa dismissed Wickremesinghe in October over political differences before being forced to reinstate him following pressure from the Supreme Court.
'I rushed downstairs fearing an earthquake'
The attacks marked the most significant violence in the country since a bloody civil war ended 10 years ago.
Six of the blasts — hitting three churches and three hotels — were reported in quick succession in the morning with the other two blasts hitting later.
St Sebastian's Church in Katuwapitiya, north of Colombo, St Anthony's Church in Kochcikade, Colombo and the evangelical Zion church in Batticaloa, in Eastern Province, were all targetted.
The first three hotels affected were the Shangri-La Colombo, Kingsbury Hotel and Cinnamon Grand Colombo.
The seventh blast occurred at a hotel in Dehiwela, a few kilometres south of capital Colombo and the eighth was at a house in the north of the capital.
Akshat Saraf, 30, from Bangalore in India, told NBC News his family "rushed downstairs fearing an earthquake" when the Shangri-La hotel, in eastern Colombo, where they were staying, was hit by an explosion.
"What has been done is a despicable act against humanity," he added.
An improvised explosive device was located and disposed of near the country's main airport late on Sunday, while another explosion occurred on Monday, down the road from St Anthony’s Church in Kochchikade, the site of one of eight bombings. A witness told Reuters that it went off when attempted to defuse it.
Three police officers were also killed during a raid on a house in Colombo after the attacks.
The government responded immediately by imposing a curfew and shutting down access to social media and messaging services. The measures, Wickremesinghe said at the time, were aimed at curbing the spread of misinformation. Schools were also closed for two days.
A state of emergency was then declared. Sirisena said the measure would be confined to dealing with terrorism and would not impinge on the freedom of expression.
Which foreign nationals were killed in the attacks?
At least 34 foreigners were among the dead, government officials said.
Eight British nationals were killed in the attacks, according to the Sri Lankan High Commissioner to the UK.
British High Commissioner to Sri Lanka, James Dauris, wrote on Twitter that he had visited "Brits in hospital who have been affected".
Denmark's richest man Anders Holch Povlsen and his wife lost three of their four children in the Easter Sunday attacks in Sri Lanka, a spokesman for Povlsen's fashion firm confirmed.
Povlsen is the majority stakeholder in online retailer Asos while also holding a big stake in Zalando.
One Portuguese national, a Dutch citizen and three Indians also lost their lives.
Chinese and Turkish state media also reported their citizens were among the victims.
The German embassy in Colombo also announced on Twitter that it is "working hard to find out whether German citizens are affected," describing the situation as "confusing."
It also warned that those departing the island should be at the airport no later than four hours before departure due to tightened security measures.
US Secretary of State Mike Pompeo said US nationals were among the dead but did not provide details.
'We are very sorry'
Some mass funerals for the victims were held on Tuesday.
The government has pledged 1 million Sri Lankan Rupees (roughly €5,102) would go to each of the families of those killed in the blasts, as well as 100,000 Sri Lankan Rupees (roughly €510) to go towards funeral expenses.
"As a government, we are responsible for that," Senaratne said. "We are very sorry."
The wounded are to receive 100,000-300,000 Sri Lankan Rupees (roughly €510-€1,530).
Watch the full news conference here:
'Government discrimination persists'
The Buddhist-majority country emerged from a bloody civil war against Tamil separatists a decade ago during which explosions in the capital were common but a lull in such attacks had since been observed.
Christians represent 7.6% of the island's population of 22 million, according to the 2012 census, behind Muslims (9.7%), Hindus (12.6%) and Buddhists (70%).
Last year, there were 86 verified incidents of discrimination, threats and violence against Christians, according to the National Christian Evangelical Alliance of Sri Lanka (NCEASL), which represents more than 200 churches and other Christian organisations.
This year, the NCEASL recorded 26 such incidents, including one in which Buddhist monks allegedly attempted to disrupt a Sunday worship service, with the last one reported on March 25.
The US State Department report into human rights practices in Sri Lanka released in April 2018 found that government discrimination towards nondenominational Christian groups persisted.
"Sources stated some Buddhist monks regularly tried to close down Christian and Muslim places of worship on the grounds they lacked the Ministry of Buddha Sasana's approval," the report detailed.