The Philippine authorities vowed to crush those behind twin bombings that killed at least 20 people and injured over 80 who had been attending a Sunday mass in the country’s restive southern island of Mindanao.

The attack came just six days after a referendum on autonomy for the mainly Muslim region returned an overwhelming “yes” vote, and was one of the deadliest in recent years in an area long plagued by conflict, insurgencies, Islamist terrorist activities and military crackdowns.

The first explosion occurred inside the Cathedral of Our Lady of Mount Carmel in Jolo, the capital of Sulu province, while the second blast struck outside near the entrance as panicked worshippers fled and security forces arrived to help.

The death toll, which was initially reported as 27 but later revised down to 20, included at least five security personnel as well as multiple civilians.