Government spokesman Rajitha Senaratne said Monday that there had been "several warnings from foreign intelligence agencies about the impending attacks" and that the NTJ had been mentioned in those warnings.
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While the NTJ is based in Sri Lanka, officials there say they believe the group had help from outside of the country.
"We don't see that only a small organization in this country can do all that," Senaratne 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."
Who is NTJ and where did they come from? Here’s what we know about the organization now.
Who are they?
The group is believed to have broken off from the Sri Lanka Thowheed Jamath, an Islamist group in the country. The SLTJ is known for inciting hatred against Buddhists. The secretary of the group - Abdul Razik – was arrested in 2016 for insulting religion and "angering a religious devotee." Razik later apologized for what he said about Buddhists.
What does Thowheed Jamath mean?
The name roughly translates as National Monotheism Organization, according to the Washington Post.
Who is the group’s leader?
The alleged leader is Mohamed Zaharan, according to Indian intelligence agencies.
Is there a large Islamic presence in the nation?
No, Muslims make up on 9.7 percent of Sri Lanka's population, according to the CIA World Fact Book.
Were they on authorities radar for possible terror activities?
The NTJ was mainly known for vandalizing Buddhist statues, prior to Sunday’s attack.
However, a little more than a week ago, police issued a three-page intelligence report to security officials in the country that radical Islamic groups, including NTJ, planned to attack Catholic Churches.
Senaratne called for the resignation of the police inspector general, Pujith Jayasundara, for failing to stop the attacks.
Where is Sri Lanka?
Sri Lanka is an island off the southern tip of India.