MANILA, Philippines (AP) — A powerful explosion believed caused by a bomb ripped through a Catholic Mass and killed at least three people and wounded several others Sunday in a predominantly Muslim city in the southern Philippines, officials said.

The morning Mass was underway in a gymnasium at the state-run Mindanao State University in Marawi city when the explosion happened, causing panic among dozens of worshippers and leaving the victims bloodied and sprawled on the ground, said Taha Mandangan, the security chief of the sprawling state-run campus.

At least two of the wounded were fighting for their lives, Mandangan said.

“This is clearly an act of terrorism. It’s not a simple feud between two people. A bomb will kill everybody around,” Mandangan told The Associated Press by telephone.

Army troops and police immediately cordoned off the area and were conducting an initial investigation and checking security cameras for any indication of who may have been responsible for the attack. Security checkpoints were set up around city.

There was no clear indication yet who was responsible for the explosion but police said they would check the possible involvement of Muslim militants, who still have a presence in the region despite years of military and police offensives.

Regional police director Brig. Gen. Allan Nobleza said investigators were assessing if the explosion was caused by a homemade bomb or a grenade, and if the attack was connected to the killing of 11 suspected Islamic militants in a military offensive backed by airstrikes and artillery fires on Friday near Datu Hoffer town in southern Maguindanao province.

Nobleza said the slain militants belonged to Dawlah Islamiyah, an armed group which had aligned itself to the Islamic State group and still has a presence in Lanao del Sur province, where Marawi city is located.

The mosque-studded city came under attack from Islamic militants aligned with the Islamic State group in 2017, leaving more than 1,100 killed, mostly militants, before the five-month siege was quelled by Filipino forces backed by airstrikes and surveillance planes deployed by the United States and Australia.

The southern Philippines is the homeland of minority Muslims in the predominantly Roman Catholic nation and the scene of decades-old separatist rebellion.

The largest armed insurgent group, the Moro Islamic Liberation Front, signed a 2014 peace deal with the government considerably easing decades of fighting. But a number of smaller armed groups rejected the peace pact and press on with bombings and other attacks while evading government offensives.