NTJ is believed to have split from Sri Lanka Thowheed Jamath (SLTJ), another hardline group. SLTJ’s leader Abdul Razik was arrested in 2016.
NTJ is being considered as a fringe group of the minority Muslim community which constitutes only 9.7 percent of Sri Lankan population.
The island has been witnessing sweeping anti-Muslim bigotry mainly fed by majority Buddhist nationalists but it never had a history of Mulsim militants.
Investigators said seven suicide bombers took part in the attacks while a government spokesman said an international network was involved.
Police had received a tip-off of a possible attack on churches by a little-known domestic Islamist group some 10 days ago, according to a document seen by Reuters.
The intelligence report, dated April 11 said a foreign intelligence agency had warned authorities of possible attacks on churches by the leader of the group, the National Thawheed Jama'ut. It was not immediately clear what action, if any, was taken to address the reported threat.
Police said 24 people had been arrested, all of whom were Sri Lankan, but they gave no more details. "Still the investigations are going on," Welianga said.
Cabinet spokesman Rajitha Senaratne said an international network was involved but did not elaborate. "We do not believe these attacks were carried out by a group of people who were confined to this country," Senaratne said. "There was an international network without which these attacks could not have succeeded."
Sri Lanka invoked emergency law which gives police and the military extensive powers to detain and interrogate suspects without court orders, which will go into effect at midnight Monday, the president's office said.
Colombo, the seaside capital of the Indian Ocean island, was jittery on Monday. Police said 87 bomb detonators were found at the city's main bus station, while an explosive went off near a church where scores were killed Sunday when bomb squad officials were trying to defuse it.
A night curfew will go into effect at 8 p.m., the government announced.
President Maithripala Sirisena said in a statement the government would seek foreign assistance to track the overseas links.
There were fears the attacks could spark communal violence, with police reporting late Sunday there had been a petrol bomb attack on a mosque in the northwest and arson attacks on two shops owned by Muslims in the west.
Sirisena fired the premier last year and installed opposition strongman Mahinda Rajapaksa in his stead. Weeks later, he was forced to re-instate Wickremesinghe because of pressure from the Supreme Court but their relationship is still fraught as a presidential election nears.