Government spokesman Rajitha Senaratne said investigators were looking at whether the National Thowheeth Jama'ath (NTJ) group had "international support" for the deadly Easter Sunday attacks on churches and luxury hotels.
Wary of stirring ethnic and religious tensions, officials have provided few details about 24 people arrested since the attacks.
Not much is known about the NTJ, but documents seen by Agence France-Presse (AFP) show Sri Lanka's police chief issued a warning on April 11, saying a "foreign intelligence agency" had reported the group was planning attacks on churches and the Indian high commission.
The group has previously been linked to the vandalizing of Buddhist statues.
"We don't see that only a small organization in this country can do all that," said Senaratne.
"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."
The death toll from the attacks on Sunday, April 21, rose dramatically Monday to 290 – including dozens of foreigners – in the country's worst attacks for over a decade.
More than 500 people were injured in the assault that saw suicide bombers hit 3 high-end hotels popular with foreign tourists, and 3 churches, unleashing carnage in Colombo and beyond.
Two additional blasts were triggered as security forces carried out raids searching for suspects.
And as tension remained high, police reported a fresh explosion as they attempted to defuse another suspected bomb found Monday near one of the 3 churches targeted. There were no further details, but police earlier also reported finding 87 bomb detonators scattered on the ground at a bus station and a nearby garbage dump.
State of emergency
President Maithripala Sirisena's office said a state of emergency "limited to counter terrorism regulations only" would be introduced from midnight Monday (1830 GMT).
"This is being done to allow the police and the 3 forces to ensure public security," the statement said, referring to the army, navy and air force.
Officials said Sirisena would meet with Colombo-based diplomats Tuesday to seek international assistance in the investigation.
"The intelligence sections have reported that there are international terror groups which are behind the local terrorists," the statement said. "International assistance will be sought to combat them."
The government information department said a new curfew would run from 8:00 pm (1430 GMT) on Monday until 4:00 am on Tuesday.
The US State Department, meanwhile, warned of further attacks in a revised travel advisory, urging increased caution and adding: "Terrorist groups continue plotting possible attacks in Sri Lanka."
The attacks were the worst ever carried out against Sri Lanka's small Christian minority, who make up just 7% of the 21 million population.
At least 37 foreigners were among the dead, citizens of India, Britain, Turkey, Australia, Japan and Portugal, as well as a dual US-British passport holder.
Memories of civil war
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.
The churches targeted included St Sebastian's in Negombo, north of the capital, which was surrounded by security forces on Monday.
Dozens of people were killed at the church, including friends of 16-year-old Primasha Fernando, who was at her home nearby when the suicide bomber struck.
"When I got to the church there were people crying and screaming," she told AFP.
"I saw bodies everywhere," she added in tears. "I saw parents carrying their dead babies. I saw dead people who had hair but didn't have faces anymore."
Many of the dead from the attacks have been taken to the government morgue and relatives on Monday begin the gruesome task of identifying their loved ones.
Eighteen bodies were released after relatives recognized a victim from video shown on a screen in a corner of the morgue courtyard.
Janaka Shaktivel, 28, father of an 18-month-old son, sat in shock outside the building waiting for the body of his wife to be handed over.
"I recognized her body from the wedding ring that she always wore," he said. "I have no words to explain my feelings."
Two leading Muslim groups issued statements condemning the attacks, with the All Ceylon Jamiyaathuul Ulama, a council of Muslim theologians, urging the "maximum punishment for everyone involved in these dastardly acts."
For many, the blasts stirred painful memories of Sri Lanka's civil war, when bomb attacks were a frequent occurrence.
"The string of blasts brings back memories of the time when we were afraid to go in buses or trains because of parcel bombs," said Malathi Wickrama, a street sweeper in Colombo.
The attacks drew global condemnation, including from US President Donald Trump and the pope. – Rappler.com