MANILA, Philippines – A “massive” manhunt is underway for the attackers who bombed a Catholic Mass that killed four people in the southern Philippines, a senior military officer said on Monday, December 4, vowing to do everything possible to bring the perpetrators to justice.

Police are looking into at least two suspects they believe were behind the blast, which Islamic State militants claimed.

“As we speak, there’s an ongoing massive operation to hunt these terrorist groups or suspected perpetrators of the bombing,” William Gonzales, Western Mindanao Command chief, told reporters after a security meeting in Marawi city.

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 an 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.

Three of the fatalities in the blast have been identified as June Ray Barbante, Evangelin Aromin, and Riza Ramos. One female victim has not been identified yet.

The United States condemned the “horrific terrorist attack” and said it stood with Filipinos in rejecting violence, joining a chorus of support from countries including 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 the bombing at Mindanao State University, shortly after Philippine President Ferdinand Marcos Jr said “foreign terrorists” were responsible.

Marawi is in an 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, who attended a security briefing with troops and police in Marawi, said he suspected the bombing could be a retaliatory attack for operations against local extremist groups in the southern region of Mindanao.

“That could be one of the strong possibilities why this occurred,” Brawner said. “Your security forces…are working doubly hard to make sure that the perpetrators of this terrorist attack will be brought to justice.”

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 State’s 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.

Spokesperson Xerxes Trinidad said the military was trying to validate it Islamic State’s claim of responsibility.

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. – with reports from Ferdh Cabrera/Rappler.com