Little-known National Thowheed Jamath behind Lanka massacre?

However, neither the NTJ, nor any other group, has admitted carrying out the wave of bombings which tore through the island nation, killing 290 people and injuring nearly 500 others.

Until Monday, when the Sri Lankan government spokesman mentioned their name, very few people had heard of the NTJ.

Rajitha Senaratne, also the health minister, said that all suicide bombers involved in the blasts are believed to be Sri Lankan nationals.

A total of seven suicide bombers carried out a series of eight devastating blasts that tore through three churches and luxury hotels in Sri Lanka on Easter Sunday, the Government Analyst's Department said on Monday.

The group is believed to have splintered off from another hardline Islamist group in the country, the Sri Lanka Thowheed Jamath (SLTJ), the BBC reported.

While still relatively unknown, the SLTJ is a bit more established. Their secretary, Abdul Razik, was arrested in 2016 for inciting hatred against Buddhists, the majority community in Sri Lanka. He later issued an apology.

Some reports have also linked the NTJ to a spate of vandalism last year that targeted Buddhist temples in Mawanella, central Sri Lanka.

But they are an extremist fringe group within a small religious minority - only 9.7 per cent of Sri Lanka's population of about 21 million are Muslim.

Their social media presence is also sparse. Although they have a Facebook page, it is only updated every few weeks or so, the report said.

Their Twitter page has not been updated since March 2018, it said.

The group's website is also offline - although it's not clear if it was taken down before or after Sunday's attacks.

Government spokesman Senaratne told reporters on Monday that there had been "several warnings from foreign intelligence agencies about the impending attacks".

Sri Lankan telecommunications minister Harin Fernando tweeted a document that was reportedly sent by Sri Lanka's police chief earlier this month.

In that, the NTJ are explicitly named - as well as a warning that the group was planning to attack churches and the Indian High Commission.

The document also names Mohamed Zahran, the group's leader.

Alan Keenan, Sri Lanka director for the International Crisis Group, also told BBC 5Live that NTJ "appears to be the same group" as those behind the Mawanella vandalism.

"The police eventually arrested a group of young men who were said to have been the students of a preacher who's named in the intelligence document that came out yesterday [Sunday]," he added.

"We don't see that only a small organisation 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."

And while not naming the NTJ directly, the Sri Lankan president's office echoed this belief that whichever group was behind the attacks had help from abroad.

"The intelligence sections have reported that there are international terror groups which are behind the local terrorists," a statement from President Maithripala Sirisena said.

"International assistance will be sought to combat them," he said.

Meanwhile, Site Intelligence Group, an American company that tracks online activity of white supremacist and jihadist organisations, reported on Monday that an Islamic State (IS) terror group supporter gave pictures of three of the alleged "commandos" involved in the Easter Day bombings in Sri Lanka.