The attacks, using two improvised explosive devices, occurred about 8:15 a.m. Sunday, just before Mass, according to a statement from Philippine Defense Secretary Delfin Lorenzana. The first bomb went off as churchgoers were attending a service, and a second was detonated as soldiers rushed into the church to respond.

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Philippine armed forces have been placed on high alert, securing “all places of worship and public places,” Lorenzana added in the statement.

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Photos of the church, the Cathedral of Our Lady of Mount Carmel, shared on social media show shattered wooden pews and debris littering the floor. Bodies were lying in the street outside.

Police said at least 20 people were killed, among them five soldiers who were responding to the first blast, revising an earlier estimate that officials said was incorrect due to double counting. At least 81 people were reported injured. Some of the casualties had to be airlifted to the nearby city of Zamboanga for treatment.

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“We are in close coordination with [Philippine National Police] counterparts to aid in the investigation and in the identification of the perpetrators,” the Philippine army said.

Later Sunday, the Islamic State’s news agency Amaq said the group claimed responsibility for the bombings, according to Reuters news service. Philippine President Rodrigo Duterte condemned the bombing as an “act of terrorism.”

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“The Armed Forces of the Philippines will rise to the challenge and crush these godless criminals,” presidential spokesman Salvador Panelo said in a statement.

Analysts say, however, that the attacks bear the hallmark of the Abu Sayyaf Group, which has long been active in Jolo and is known to have its base of operations in the province. The militant group has been listed as a foreign terrorist organization by the U.S. State Department. Various abductions, bombings and killings have been attributed to it, including the 2001 kidnapping of three American citizens.

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The Abu Sayyaf Group “has the capability to mount such kinds of mass casualty bombing attacks,” said Rommel Banlaoi, chairman of the Philippine Institute for Peace, Violence and Terrorism Research.

The attacks come just after the region voted this week in a landmark referendum aimed at bringing peace to an area that has for decades been plagued by conflict and violence.

The vote for the so-called Bangsamoro Organic Law, ratified this week, created an autonomous region in the Muslim-heavy region of Mindanao, which will be governed by a transitional authority before a regional government is set up.

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Jolo, the capital of Sulu province, rejected the law but would still be considered part of the autonomous region. Analysts say it will be extremely difficult to implement the law in that part of the region.

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Some fear the blast may be a way to shake up the peace process, as some militant factions were not involved in negotiations for the Bangsamoro. It also comes after a mall bombing that left two dead in Cotabato City, about 370 miles away, in December. A military clash with Islamic State-inspired fighters in the province of Lanao del Sur also left three militants dead earlier this week, according to local media reports.

Jun Esperon, the Philippines’ national security adviser, said authorities have not yet determined whether the two explosions are related to the vote on the Bangsamoro law.

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“The perpetrators are mass murderers. . . . We will not allow them to spoil the preference of the people for peace,” he said.

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Lorenzana urged the public to “remain calm and avoid spreading panic in our respective communities, to deny terrorism any victory.”

The Catholic Bishops Conference of the Philippines issued a statement condemning the attacks and appealed for peace among all religions in the region.

“As we begin a new phase in the peace process with the creation of [the autonomous region], we ask our Christian brethren to join hands with all peace-loving Muslim and Indigenous People communities in the advocacy against violent extremism,” the statement said.