Sri Lankan officials failed to heed warnings from intelligence agencies about the threat of an attack, a minister claims.
International intelligence agencies warned of the attacks several times starting April 4, Health Minsiter Rajitha Senaratne said.
On April 9, the defense ministry wrote to the police chief with intelligence that included the group's name, he said.
Two days later, according to Senaratne, police wrote to the heads of security of the judiciary and diplomatic security division.
It was not immediately clear what action, if any, was taken in response.
Authorities said little was known about the group except that its name had appeared in intelligence reports.
Because of political dysfunction within the government, Seranatne said, Prime Minister Ranil Wickremesinghe and his Cabinet were kept in the dark about the intelligence until after the attacks.
President Maithrela Sirisena, who was out of the country at the time of the attacks, ousted Wickremesinghe in late October and dissolved the Cabinet.
The Supreme Court eventually reversed his actions, but the prime minister has not been allowed into meetings of the Security Council since October.
While inspecting it, police found three bombs they attempted to defuse but that instead detonated. No injuries were reported.
Also on Monday, police found 87 detonators near Colombo's main bus depot, officials said.
They declined to comment on whether they were linked to Sunday's attacks.
Two government ministers had alluded to intelligence failures.
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.”
Mano Ganeshan, the minister for national integration, said the security officers within his ministry had been warned by their division about the possibility two suicide bombers would target politicians.
Police spokesman Ruwan Gunasekara said the Criminal Investigation Department investigating the blasts will look into the reports.
Defence Minister Ruwan Wijewardena previously described the blasts as a terrorist attack by religious extremists, and police said 24 suspects were arrested, though there was no immediate claim of responsibility.
A curfew imposed was lifted by the Sri Lankan government on Monday morning, but many social media networks remained blocked.
Prime Minister Ranil Wickremesinghe said he feared the massacre could trigger instability in Sri Lanka, and he vowed to “vest all necessary powers with the defence forces” to take action against those responsible.
The Archbishop of Colombo, Cardinal Malcolm Ranjith, called on Sri Lanka’s government to “mercilessly” punish those responsible “because only animals can behave like that”.
The scale of the bloodshed recalled the worst days of Sri Lanka’s 26-year civil war, in which the Tamil Tigers, a rebel group from the ethnic Tamil minority, sought independence from the Buddhist-majority country. The Tamils are Hindu, Muslim and Christian.
Two Muslim groups in Sri Lanka condemned the church attacks, as did countries around the world, and Pope Francis expressed condolences at the end of his traditional Easter Sunday blessing in Rome.
“I want to express my loving closeness to the Christian community, targeted while they were gathered in prayer, and all the victims of such cruel violence,” Francis said.
Six nearly simultaneous blasts took place in the morning at the shrine and the Cinnamon Grand, Shangri-La and Kingsbury hotels in Colombo, as well as at two churches outside Colombo, according to a Sri Lankan military spokesman, Brigadier Sumith Atapattu.
A few hours later, two more blasts occurred just outside Colombo, one of them at a guesthouse, where two people were killed, the other near an overpass, Atapattu said. A further three police officers were killed in a search of a property related to the bombings.