Media reported that three people were killed and nine injured when an explosion occurred in a gymnasium at a university in southern Philippines during a Catholic mass on Sunday. Regional police director Brigadier General Allan Nobleza told reporters that authorities are investigating the explosion that took place at Mindanao University. He added that the attack could be a retaliatory action by militants linked to ISIS, which will be explored in the investigation along with other aspects. The explosion occurred in Marawi city, which was besieged by ISIS-affiliated militants for five months in 2017. The explosion followed hours after the Philippine military announced it had killed 11 militants, including members of an ISIS-affiliated group, in a military operation in Maguindanao province. Mindanao University stated on Facebook that it is "deeply saddened and horrified by the violence that occurred during a religious gathering. We unequivocally and strongly condemn this horrific and senseless act."