But in recent years some Buddhist monks have turned militant and incited followers to attack Muslims, their places of worship and some of their businesses, such as slaughterhouses. The Sri Lankan government’s security services appeared to have turned a blind eye, allowing Buddhist mobs to act with impunity.

In 2014, scores were injured and three people were killed in Buddhist-Muslim clashes. In response, some Muslims joined radical Islamist groups that they believed would defend their faith.

According to the April 11 security memo, National Thowheeth Jama’ath’s leader, Mohammed Zaharan, had been under close watch for several days. Sri Lankan security officials have blamed his group or allied groups for vandalizing Buddhist statues in December, a serious crime that was seen as an attempt to instigate bloodshed between Buddhists and Muslims.

But in January, Sri Lankan officials said that evidence had emerged revealing that homegrown Islamist groups were even more dangerous. Investigations connected to the statue destruction led police officials to a farm in northwestern Sri Lanka where officers discovered a weapons cache with more than 100 kilograms of explosives, detonators, wire cords, a rifle, bullets, dry rations and religious propaganda.

Sri Lankan officials have since said that the cache belonged to a radical Islamist group, probably one connected to National Thowheeth Jama’ath. But several security specialists said it was unlikely that National Thowheeth Jama’ath members could have carried out the bombings on their own.

The group had never attempted such a devastating, coordinated attack, with numerous suicide bombers striking different places nearly simultaneously.

“The target selection and attack type make me very skeptical that this was carried out by a local group without any outside involvement,” said Amarnath Amarasingam, a specialist in Sri Lankan extremism at the Institute for Strategic Dialogue, a counterterrorism research group in London. “There’s no reason for local extremist groups to attack churches, and little reason to attack tourists.”