MANILA (UPDATED) — The Philippine National Police on Monday said it was looking into at least two persons of interest who could be linked to the bombing of a Catholic mass that killed four people and injured dozens at the Mindanao State University in Marawi.

The 2 persons of interest in Sunday's blast were involved in previous bomb attacks in other parts of Mindanao, PNP spokesperson PCol. Jean Fajardo said.

"Base sa mga nakalap na ebidesya, including yung witnesses accounts, mayroon na silang tinitingnan na 2 persons of interests at base doon sa mga pinakita nilang larawan ay may mga positive identification doon sa POIs (persons of interest)," she said.

The Islamic State group has claimed responsibility for the MSU explosion in Marawi, the country's largest Muslim city, which was besieged by militant groups in 2017.

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Security officials had earlier said the blast from an improvised explosive device (IED) might have been a retaliatory attack for a series of military operations against militant groups in recent days.

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Marawi, the country's largest Muslim city, was besieged in 2017 by pro-Islamic State Maute and Abu Sayyaf militants, including foreign and local fighters.

"Doon sa possible involvement ng foreign terrorists, alam natin previously may mga nare-report tayong presence ng foreign terrorist individuals diyan sa ibang area ng Mindanao," Fajardo said in a press briefing.

"So it may be a possibility based sa ishinare sa atin ng SND (Secretary of National Defense), but on the part of the PNP we will be basing our investigation based on the evidence na nako-collate natin including mga witnesses accounts," she added.

There is no indication that the bombing was a suicide attack, the official added.

She said authorities recovered fragments of the IED and were still looking into whether it was battery or cellphone operated. They also found a bag where the bomb could have been hidden.

"Wala silang narecover na mga CCTV footages that might help them in identifying the possible suspects, but doon sa labas ng MSU nagkakalap sila ng mga footages para makatulong sa ginagawang imbestigation," Fajardo added.

The PNP has not monitored any serious threat in Metro Manila following the Marawi blast.

— With reports from Bianca Dava and Raya Capulong, ABS-CBN News; Agence France-Presse

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