MANILA: Three people were killed and nine wounded when an explosion ripped through a Catholic Mass in a university gymnasium in the southern Philippines on Sunday, police said.

The authorities are investigating the explosion at Mindanao State University, the regional police director, Brigadier General Allan Nobleza, told reporters, adding that one angle being pursued was possible revenge by pro-Islamic State militants.

The blast occurred Marawi, a city besieged by militants in for five months in 2017.

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The Philippine military said on Saturday they had killed 11 militants, including members of the Dawlah Islamiyah-Philippines, a pro-Islamic State group, in a military operation the day before in Maguindanao del Sur province.

“I condemn the violent bombing incident that transpired this morning,” Lanao del Sur Governor Mamintal Adiong Jr said in a statement.

“Terroristic attacks on educational institutions must also be condemned because these are places that promote the culture of peace.”

Mindanao State University is “deeply saddened and appalled by the act of violence that occurred during a religious gathering,” it said in a statement on Facebook.

“We unequivocally condemn in the strongest possible terms this senseless and horrific act.” The university said it was said it was suspending classes until further notice.