The Islamic State claimed responsibility on Sunday for the bombing of a Catholic church in the Philippines that killed at least 20 people and wounded 111 others. Some estimates put the number of people killed in the attack as high as 27.

The attack was perpetrated at the Cathedral of Our Lady of Mount Carmel in the southern island province of Sulu during Sunday morning mass. Sulu is a Muslim-majority province where the terrorist organization Abu Sayyaf has long been active. Abu Sayyaf is now allied with ISIS and opposes the dominant Islamist group in the Philippines, the Moro Islamic Liberation Front (MILF).

The bombing was conducted with classic ISIS tactics, using a double-bomb approach that has been employed many times in the Middle East against mosques that practice the “wrong” kind of Islam: an initial blast inside the church that sent terrified congregants scrambling for the exit, where a second, deadlier bomb awaited them. Police believe the second bomb was hidden in a motorcycle parked outside the church.

“The initial explosion scattered the wooden pews inside the main hall and blasted window glass panels, and the second bomb hurled human remains and debris across a town square fronting the cathedral,” Fox News reported on Monday.

Fox reported armored personnel carriers sealing off the area while the many dead and wounded were driven to local hospitals or airlifted to more distant facilities. The Philippine government responded to the attack with cold fury:

“I have directed our troops to heighten their alert level, secure all places of worships and public places at once, and initiate pro-active security measures to thwart hostile plans,” Defense Secretary Delfin Lorenzana said.

“We will pursue to the ends of the earth the ruthless perpetrators behind this dastardly crime until every killer is brought to justice and put behind bars. The law will give them no mercy,” the office of President Rodrigo Duterte said in Manila.

It added that “the enemies of the state boldly challenged the government’s capability to secure the safety of citizens in that region. The (Armed Forces of the Philippines) will rise to the challenge and crush these godless criminals.”

Asia Times speculated on Sunday that the church bombing could be an ISIS pushback against the referendum to create the BARRM, an almost completely autonomous Muslim region in the largely Catholic Philippines that would be administered by the MILF, effectively establishing an Islamic state controlled by rivals of the Islamic State:

Sulu voted against inclusion in the BARMM, which will replace the 28-year-old Autonomous Region in Muslim Mindanao (ARMM) which was the outgrowth of a separate peace deal with a different Islamic Moro rebel group.

Sulu Governor Abdusakur Tan II has challenged the legality of the BOL before the Supreme Court, arguing that its enactment is unconstitutional because it abolishes the existing ARMM, which he argues can only be done by amending the Constitution and not through an act of Congress.

The BARMM aims to bring just and lasting peace to the region after four decades of civil war that has impoverished the Muslim region, the country’s perennially poorest.

While President Rodrigo Duterte, who hails from Mindanao, has yet to install the BARMM, fighting erupted on Thursday between the military and remnants of the Islamic State-aligned Maute Group that laid siege to Mindanao’s Marawi City beginning in May 2017.

From this perspective, the church bombing might have been retaliation for Philippine military action against ISIS-aligned terrorists that killed three fighters and captured three others late last week. Peace activist Octavio Dinampo, who was once kidnapped by Abu Sayyaf, told the Asia Times the “brutal and grisly” attack on the cathedral was “an act of reprisal and plain and simple terrorism to sow fear among civilians.”

Asian Times quoted a representative of the MILF condemning the church attack as a “terrible act of terrorism.”

The ISIS claim of responsibility arrived in a formal communique issued after the terrorist organization’s Amaq news agency described the attack as an Islamic State operation. Philippine security forces more specifically pinned the bombing on an Abu Sayyaf splinter group called “Ajang-Ajang” that has demonstrated a troubling ability to carry out terror attacks in high-security environments. Ajang-Ajang is noted for being supportive of ISIS.

Philippine officials pointed out that some of the details in the Islamic State’s claim of responsibility were not accurate – for example, ISIS referred to suicide bombers wearing explosive vests but the explosion outside the church emanated from a parked vehicle.