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 310 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 statement posted to Twitter.
"It is imperative that we remain unified as Sri Lankans in the face of this unspeakable tragedy," he added.
- Source: News websites