I read the article ... it doesn’t say.
According to wiki (hah!) mohammedans are about 3% of the population of Burundi. It could have been them, it could also have some connection to the Hutu/Tutsi tribal hate-fest.
Maybe you’ll consider the CIA a more reliable source:
Catholic 62.1%, Protestant 23.9% (includes Adventist 2.3% and other Protestant 21.6%), Muslim 2.5%, other 3.6%, unspecified 7.9% (2008 est.)
https://www.cia.gov/library/publications/the-world-factbook/geos/by.html
It could be random sicko perverts. We have them here, too.