Scrutiny fell on rifts in Sri Lanka's leadership on Monday after it emerged that authorities were warned about the group accused in Sunday's bombings.
Attacks on churches and hotels killed 290 people and wounded nearly 500.
Security agencies had been watching the National Thowheed Jamath jihadist group, reports said, and had notified police about a possible attack.
But the prime minister, Ranil Wickremesinghe, and the cabinet were not informed, ministers said.
On Tuesday, the country is observing a day to "mourn the senseless loss of innocent lives," Mr Wickremesinghe said in a tweet.
"It is imperative that we remain unified as Sri Lankans in the face of this unspeakable tragedy," he added.
A 'colossal intelligence failure'
The prime minister was not privy to security briefings following a rift last year with President Maithripala Sirisena, cabinet spokesman Rajitha Senaratne told a press conference.
Mr Sirisena sacked Mr Wickremesinghe and his cabinet in October and tried to install another prime minister, prompting a full-blown constitutional crisis.
He was forced to reinstate Mr Wickremesinghe under pressure from the nation's supreme court, but apparently kept the prime minister from security briefings.
Mr Senaratne said intelligence agencies had begun issuing warnings about the group on 4 April, after which the defence ministry sent a detailed warning to the chief of police; and on 11 April a memo was sent to the heads of several security divisions.
He said information passed to police included a warning from a foreign intelligence agency about possible attacks by the group, as well as names of members.
US media quote Sri Lankan officials as saying both Indian and US intelligence had warned the country about a threat in early April.
It was not clear on Monday whether Mr Sirisena had been made aware of the warnings. "Our understanding is that it was correctly circulated among security and police," Shiral Lakthilaka, a senior adviser to Mr Sirisena, told the BBC.
He said that the president had appointed a special committee led by a supreme court judge to investigate what had happened.
Rauff Hakeem, the minister for town planning, called the attacks a "colossal failure of intelligence".
"It is shameful for all of us. We are all ashamed of this," he said.
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."
What steps has the government taken?
Sri Lankan authorities declared a state of emergency which came into effect at midnight (18:30 GMT) on Monday.
The declaration gives police and the military sweeping powers to detain and interrogate suspects without court orders - powers that were last used during the nation's civil war.
A curfew also came into effect at 20:00, leaving armed security forces patrolling largely deserted streets in Colombo on Monday night.
The government blocked access to Facebook, WhatsApp and Instagram after the blasts.
Police arrested 24 suspects in a series of raids just hours after the attacks, and later said 87 bomb detonators had been found at the Bastian Mawatha private bus station in Petta.