MANILA - Pope Francis on Sunday sent a message of assurance to the victims of the deadly blast in Marawi City, which happened during a mass inside Mindanao State University.

“I’d also like to assure the victims in the Philippines where there was a bomb that exploded during mass. I am very close to the families of Mindanao who have suffered so much because of this,” the Pope said in his message.

• At least 4 killed after explosion rocks Mindanao State University in Marawi

Cardinal Orlando B. Quevedo, member of the Council of Leaders in the Bangsamoro Autonomous Region in Muslim Mindanao, also sent his condolences to the families of the victims.

“The massacre at the Mindanao State University Gym in Marawi City during Sunday Mass this morning is a crime that literally cries out to heaven. Perpetrated on the 1st Sunday of Advent, a season of hope, and at the beginning of the Mindanao Week of Peace, the massacre is the most terrible and most damnable terroristic crime against innocent worshippers on a Christian holy day,” he said in a statement.

“It is a tragic re-enactment of the insane bombing in the Jolo Cathedral during Sunday Mass several years ago. I call upon our law enforcement agencies to ferret out the perpetrators of this heinous crime and bring to justice. For the victims and for their grieving families, my deep personal condolences and prayers. Let peace begin in our hearts,” he added.

At least 4 people were killed, while 9 others were injured in the explosion.

More than 40 people were brought to government hospital Amai Pakpak Medical Center following the deadly explosion. Six of them were inside the operating room.

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Authorities have yet to determine the motive behind the explosion, but they are looking at the possibility that it could be a retaliatory attack following a series of military operations against 3 terrorist groups in Mindanao.

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Lanao del Sur and Maguindanao del Sur are part of the Bangsamoro Autonomous Region in Muslim Mindanao.

The country's Muslim minority were given fuller autonomy under former president Rodrigo Duterte as part of efforts to head off the lure of violent extremism.

Bangsamoro Government Chief Minister Ahod Ebrahim said he "condemned the atrocious and cowardly acts" and called for a "thorough investigation" into the incident.

Militant attacks on buses, Catholic churches, and public markets have been a feature of decades-long unrest in the region.

Manila signed a peace pact with the nation's largest rebel group, the Moro Islamic Liberation Front, in 2014, ending their deadly armed rebellion.

But smaller bands of Muslim fighters opposed to the peace deal remain, including militants professing allegiance to the Islamic State group.

Communist rebels also operate in the region.

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-with a report from Agence France-Presse