Philippine police investigating at least two persons of interest over bombing

Philippine police have identified at least two suspects in the bombing of a Catholic Mass that killed four people, a regional police chief said on Monday, vowing to hunt down those behind the blast, which was claimed by Islamic State militants.

The bomb went off on Sunday during a service at a university gymnasium in Marawi, a city left in ruins in 2017 by a five-month military campaign to end a bloody occupation by Islamic State loyalists that had triggered alarm across Asia.

"(We have persons) of interest, but the investigation is still ongoing. In order not to preempt the investigation, we will not divulge the names," regional police chief Allan Nobleza told GMA News, adding that one of the suspects was linked to a local militant group.

The United States condemned what it called a "horrific terrorist attack" and said it stood with Filipinos in rejecting violence, joining a chorus of support from countries that include Japan, Australia, Britain, China and Canada.

"We mourn those killed in the attack, and our thoughts are with the injured," State Department spokesperson Matthew Miller said in statement.

Islamic State militants claimed responsibility for Sunday's bombing at Mindanao State University, shortly after Philippine President Ferdinand Marcos Jr said "foreign terrorists" were responsible.

Marawi is in a area known as Bangsamoro, an underdeveloped Muslim region in the predominantly Catholic Philippines, that has for decades battled with lawlessness, separatist violence and clan conflicts, prompting concern that it could become fertile ground for extremism.

Military chief Romeo Brawner on Sunday said he suspected the bombing could be a retaliatory attack as it had followed operations against local extremist groups in the southern region of Mindanao.

The military recently said it had killed a senior operative from Abu Sayyaf, a group notorious for kidnapping and piracy that has aligned itself with the Islamic State.

Abu Sayyaf's late leader, Isnilon Hapilon, was Islamic States anointed "emir" in Southeast Asia and mastermind of the 2017 occupation of Marawi, which led to the deaths of more than 1,000 people during a five-month war, among them fighters from multiple countries. Hapilon was killed by a sniper.

The military said it had also conducted two operations in Mindanao on Friday and Sunday against Dawlah Islamiya-Maute, a group that joined Hapilon in taking over Marawi in 2017, seeking to turn it into a Southeast Asian "wilayat" – or governorate - for Islamic State.

Xerxes Trinidad, a spokesperson for the military, said it was aware of Islamic State's claim of and responsibility and was trying to validate it.

Of the 54 who were wounded in the bombing, Trinidad said only seven remained in hospital.

Classes resumed in Marawi on Monday, but with tighter security.

"Though saddened by the explosion, we have to go back to normalcy and address this tragic incident," Marawi Mayor Majul Gandamra told DWPM radio station.