“Most of the bombers are well-educated, come from economically strong families. Some of them went abroad for studies,” Wijewardene told a news conference.

“One of them we know went to the UK, then went to Australia for a law degree. Foreign partners, including the UK, are helping us with those investigations.”

Two of the bombers were brothers, sons of a wealthy spice trader and pillar of the business community, a source close to the family said.

Intelligence officials and Prime Minister Ranil Wickremesinghe believe that Zahran, a Tamil-speaking preacher from the east of the Indian Ocean island country, may have been the mastermind.