Prime Minister Ranil Wickremesinghe has acknowledged that police had intelligence about the suicide strikes against churches and luxury hotels before the bombings on Sunday, but failed to act on it.
More than 300 people have been killed in the attack with over 500 injured.
An investigation was underway to establish why police did not share intelligence with Wickremesinghe's office, he told reporters on Tuesday, shortly before the president's address.
Meanwhile, The Islamic State group has claimed responsibility for the series of bombings at three Catholic churches and three high-end hotels, and released a photo of the men it said were responsible.
It came after Sri Lanka's government said initial investigations suggested the attack had been carried out as "retaliation" for shootings at two mosques in New Zealand last month that killed 50 people.
The Sri Lankan government had already pointed the figure at a little-known local Islamic extremist group called National Thowheeth Jama'ath (NTJ), but said it was investigating whether they had international support.
"Those that carried out the attack that targeted members of the US-led coalition and Christians in Sri Lanka the day before yesterday are Islamic State group fighters," a statement released by the group's propaganda agency Amaq said.
Prime Minister Wickremesinghe has acknowledged that police had intelligence about the suicide strikes but failed to act on it.