After fatal religious unrest broke out between Buddhists and Muslims in Sri Lanka in 2014, Ahmed said he was beginning to pay more attention to Mr. Zaharan. Ahmed's moderate Muslim group sought to calm tensions between belligerent communities, and at a meeting with government intelligence agents in early 2015, Ahmed said he suggested that intelligence agents monitor Mr. Zaharan.
Zaharan disappeared for a few years, Ahmed said, but last December he reappeared in the city of Mawanella in the center of Sri Lanka. Some young men, the police claimed to be Muslims, disfigured three Buddhist statues, and after they were arrested, the police officers found that the young men had taken religious courses from Mr. Zaharan.
Around the same time, Indian intelligence officials began following Mr. Zaharan. They said Tuesday they had cracked a cell of the Islamic State in southern India. A member of this cell said he had been influenced by the videos of Mr. Zaharan in which the preacher encouraged the faithful to join the Islamic State. The Indian investigation was gaining momentum.
Mr. Zaharan's small group, which he named National Thowheeth Jamaath, became much more ambitious and the Indians were increasingly worried. At the beginning of April, they gave Sri Lankans names, addresses and phone numbers of Mr. Zaharan's followers. The Indians said they had information that Mr. Zaharan was planning to blow up churches and attack the Indian Embassy in Sri Lanka. The Sri Lankans did not arrest and did not increase security in the churches.
On Easter morning, just hours before the bombings, the Indians again told the Sri Lankans that an attack was imminent, according to an Indian official. The Sri Lankans did nothing.
On Tuesday, the video released by the Islamic State appeared to show Mr. Zaharan and seven unidentified persons committing themselves to the leader of the Islamic State, Abu Bakr al Baghdadi.
. Zaharan, the only one whose face is visible, is also the only one holding an assault rifle.