MANILA — Gen. Romeo Brawner, Armed Forces chief of staff, does not discount what he said is a strong possibility that the bombing at a gymnasium in the Mindanao State University campus in Marawi City on Sunday that killed 4 people and left more than 40 others injured.

Brawner made the statement after a command conference at the 103rd Infantry Brigade of the Philippine Army and of the regional office of the Philippine National Police at Camp Ranao.

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Brawner said the attack could be revenge for Armed Forces of the Philippines operations that have resulted in the neutralization of many terrorists, including leaders of the Abu Sayyaf Group and the Dawlah Islamiyah or DI.

"We all know that the DI has links with international groups, so we are not discounting that... theory of retaliatory actions of the ISIS group," said Brawner.

"We are looking at all aspects, we are looking at the all angles. Remember, when the Marawi siege happen here, it was the Dawla Islamiyah Southeast Asia group that claimed they were in control of what was happening here," he said, stressing authorities are not ruling out ISIS involvement.

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"We are not denying or confirming that," PBGen. Allan Nobleza, Philippine National Police-Bangsamoro Autonomous Region in Muslim Mindanao regional director, said of ISIS claiming responsibility for the attack.

The police, which has formed a Special Investigation Task Group on the blast, said it also has CCTV footage that might help with the probe into the bombing.

Based on the post-blast investigation of the Explosive Ordnance Division K9 and forensic field unit, a 60mm mortar and high explosive RPG were used to create the improvised explosive device in the MSU blast.

"Wala ho nakita na circuit board or timer, (ito ay) nagsasabi na it was activated through remote control," said Nobleza.

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Investigators have also ruled out the possibility of a suicide bomber.

"It was not body-worn IED or suicide bomber," added Nobleza.

Nobleza also mentioned that they have identified a person of interest who may have been involved in the attack.