The Armed Forces of the Philippines is still verifying reports that the Islamic State of Iraq and Syria or ISIS is responsible for the explosion of an improvised explosive device at a gymnasium inside the Mindanao State University in Marawi City, resulting in the deaths of four people. 

AFP public affairs chief, Col. Xerxes Trinidad told reporters on Monday that the military has already initiated its investigation in coordination with the Philippine National Police to determine the bomb signature used in the blast at the Dimaporo Gymnasium on Sunday morning. 

Trinidad said the probe would help the security sector to identify the terrorist group behind the bombing.

The AFP, he added, is also validating the claims made by ISIS in recent news reports as well as the involvement of the Daulah Islamiyyah—Maute Group in the "heinous acts of terror."

According to the SITE Intelligence Group — a counterterrorism threat intelligence organization that tracks the online activity of extremist groups — ISIS, in a communique, claimed responsibility for the bomb attack, saying its fighters "detonated an explosive device on a large gathering of Christian disbelievers in Marawi City." 

"The AFP is on heightened alert to ensure the safety of the public and that the perpetrators responsible will be apprehended and/or neutralized," Trinidad said. 

"The AFP remains committed to its mandate of protecting the people and the state from all various threat groups, foreign and domestic," he added. 

In a press conference on Sunday, Defense Chief Gilberto Teodoro Jr. mentioned the strong indication of a foreign element involved in the blasting, without elaborating further. 

"There are strong indications of a foreign element and let's leave it at that so that the investigation by both the PNP and AFP will not be hampered," Teodoro stressed. 

AFP chief Gen. Romeo Brawner Jr., meanwhile, said the bombing could be a retaliation act after the successive military operations against the militant group Daulah Islamiyah. 

Senator Risa Hontiveros, meanwhile, said the bomb attack was "meant to sow division and distrust among Filipinos." 

"There can be no justification for this act of pure evil and hate…Our unity, our solidarity, and our love for each other will defeat this act of terror," Hontiveros said. 

The ISIS-linked Maute terrorist group was primarily responsible for a five-month-long siege in Marawi City beginning in May 2017.