During Sunday Mass, bombs exploded in the Catholic Cathedral of Jolo. Twenty people were killed and at least 81 people injured, including 14 soldiers and two policemen.
The first device went out in the cathedral and the second device was pointed nearby at soldiers who came to the aid of the victims of the first explosion, said a military spokesman.
Mindanao, an island chain in the extreme south of the Philippines on the borders of Malaysia and Indonesia, has long been plagued by terrorism and unrest.
It houses several Islamist insurgent groups, including Abu Sayyaf The blame has been set for a series of attacks on civilians and Filipino government forces, as well as for kidnapping several foreigners.
Followers of last week's referendum, which will give greater autonomy to Muslim-dominated parts of Mindanao, hope that this is possible a peaceful solution to the protracted conflict that has claimed more than 1
But while Islamist and criminal groups have been active for years in the lawless three-pronged area between the Philippines, Malaysia and Indonesia, the rise of ISIS affiliates has led to a sharp escalation of violence.
In 2017, ISIS fighters besieged the city of Marawi in Mindanao for five months. Subsequent violence forced over 350,000 residents to flee the city and surrounding areas as their homes were destroyed by government bombing.
In the 150 days of the Philippine Army ransacking the militants, there were more than 800 militants and 162 members of the government security forces were killed.
Analysts say Sunday's bombing of the Cathedral had all the hallmarks of ISIS, almost supplanted by its former strongholds in Iraq and Syria, and shows that the group is still influential in the Mindanao region.
Speaking to CNN, Sidney Jones, director of the Jakarta-based Institute for Policy Analysis of Conflict, said that ISIS is "still active" in the region, but has changed since the Marawi 2017 cast.
The Marawi siege had been such that after the end of Marawi, ISIS components would use force elsewhere, "she said," since then we have had incidents in Lanao del Sur, Basilan – the bombing of Lamitan – Cotabato and others . "
Jones said the bombing showed the readiness of Islamist extremist elements in the region, many of whom had pledged to carry out attacks for ISIS, even though ISIS was routinely crushed in the Middle East.
" I think so is a reminder that the establishment of the (autonomous region) does not eradicate extremism, "Jones said.
" These guys are marching to another drummer, they are not motivated by the founding of the (autonomous region), they see no ethnic political unity led by Maguindanaon as the goal they aspire to. "Group gaining autonomy under the proposed separation of powers.
Local, national and international figures have condemned the attack, including the Moro Islamic Liberation Front (MILF), one of the key players in the peace process.
A longtime separatist movement that has entered into dialogue with the government condemned the bombing and said it would help bring the perpetrators to justice.
Mohammad Iqbal, chairman of the MILF Peace Panel, said his organization was "ready "Supporting efforts to arrest the perpetrators of the pointless violence that took place in a place of worship while people attended the morning mass" Philippine State Media PNA.
Threat level increased & # 39; nationwide & # 39;
Meanwhile, there is growing concern that bombing in other cities in the region could trigger fatal attacks. On Monday, the Philippine National Police was set to "nationwide high alert" after the recent violence.
"It could turn out that other areas can conduct their own bombings, and other groups could bomb in Cotabato, Davao (and other major cities in Mindanao)," said Rommel Banlaoi, chairman of the Philippine Institute for Peace. Violence and terrorism research.
The attack shows that "terrorist threats in Mindanao are still real," he added.
Almost two years after the siege of Marawi, the problem of Islamic extremism continued to bring parts of Mindanao to light after martial law, which was not to expire before the end of this year.
The fact that a major attack was carried out during martial law was a "major blow" to the government's policies, Banlaoi said.
"This mass casualty bomb occurred despite martial law and during a high-security environment (set up for referendum)," he said. "Let's see if it means an extension of martial law."