COLOMBO: The Sri Lankan government believes a local Islamist extremist group called the National Thowheeth Jama'ath (NTJ) was behind the deadly suicide bomb attacks that killed nearly 300 people, government spokesman Rajitha Senaratne said Monday.
Senaratne, who is also a cabinet minister, added that the government was investigating whether the group had "international support".
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 has 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.
Meanwhile, the government said it will declare a nationwide emergency from midnight on Monday following the serial blasts.
"The government has decided to gazette the clauses related to prevention of terrorism to emergency regulation and gazette it by midnight," the Sri Lankan President Maithripala Sirisena's media unit said in a statement.
It said the measure would be confined to dealing with terrorism and would not impinge on freedom of expression.
The government also ordered a new night-time curfew as tensions remained high. The government information department said the curfew would run from 8:00 pm on Monday (local time) until 4:00 am on Tuesday.
The number of deaths from the bomb blasts that ripped through churches and luxury hotels rose dramatically Monday to 290 - including dozens of foreigners - as police announced new arrests over the country's worst attacks for more than a decade.
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.
Two additional blasts were triggered as security forces carried out raids searching for suspects.
There was no immediate claim of responsibility, but police said Monday 24 people had been arrested.
Prime Minister Ranil Wickremesinghe said an investigation would look into "why adequate precautions were not taken".
President Maithripala Sirisena, meanwhile, who was abroad at the time of the attacks, returned Monday and was chairing a Security Council meeting, his office said.