Sri Lanka government believes local Islamist group behind deadly Easter blasts
Senaratne, who is also a cabinet minister, added that the government was investigating whether the group had "international support".
"We don't see that only a small organisation in this country can do all that," he said.
"We are now investigating the international support for them, and their other links, how they produced the suicide bombers here, and how they produced bombs like this."
Documents seen by AFP show Sri Lanka's police chief issued a warning on April 11, saying that a "foreign intelligence agency" had reported NTJ was planning attacks on churches and the Indian high commission.
Not much is known about the NTJ, a radical Muslim group that his been linked to the vandalising of Buddhist statues.
A police source told AFP that all 24 people in custody in connection with the attacks belong to an "extremist" group, but did not specify further.
Security has been ramped up across Sri Lanka as security forces hunt for suspects, with 24 people arrested so far. Wary of sparking ethnic and religious tensions, the government has given few details about those detained.
The death toll from bomb blasts that ripped through churches and luxury hotels in Sri Lanka rose dramatically Monday to 290 - including dozens of foreigners.
More than 500 people were injured in the Easter Sunday assault that saw suicide bombers hit three high-end hotels popular with foreign tourists, and three churches, unleashing carnage in Colombo and beyond.
The attacks were the worst ever carried out against Sri Lanka's small Christian minority, who make up just seven percent of the 21 million population.
At least 37 foreigners were among the dead, citizens of India, the UK, Turkey, Australia, Japan and Portugal, as well as a dual US-British passport holder.
The churches targeted included St Sebastian's in Negombo, north of the capital, which was surrounded by security forces on Monday.
On Monday, top Muslim leaders in Sri Lanka on Monday demanded "maximum punishment" for the perpetrators of suicide bomb attacks on Christian churches and luxury hotels .
"We urge the government to provide security to all religious sites and to give maximum punishment to everyone involved in these dastardly acts," said the All Ceylon Jamiyyathuul Ulama, or council of Muslim theologians.
"On behalf of the Sri Lankan Muslim community we offer our condolences to the people of Christian faith and extend our hands of friendship in solidarity."
The top theologians added in a statement that they had met the Archbishop of Colombo, Cardinal Malcolm Ranjith following the attacks on three churches celebrating Easter mass.
The National Shoora Council, a group of 18 Muslim organisations also expressed condolences and said the government must not "leave any stone unturned in its efforts to apprehend the culprits whoever they may be and to whatever part of the populace they may belong to".
Ethnic and religious violence has plagued Sri Lanka for decades, with a 37-year conflict with Tamil rebels followed by an upswing in recent years in clashes between the Buddhist majority and Muslims.
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