Looks like it might well be Islamists.
Looks like it. I did a little reading about the country a while back. Lots of violence between Hindus and Buddhists. 100,000 killed over a quarter century in what's not a very big country.
I didn't even know they had Muslims there. For years if you heard about bombs or violence in Sri Lanka it wasn't about Muslims. But an attack on Christians makes it something different from what was usually happening.
This could have been done by a group from outside the country or with ties outside the country, because the Muslims who live there have more quarrels with the majority Buddhist population than with the Christians.