In video messages, the militants demanded $80 million for the hostages' release, with a deadline set for April 25. Both Canada and the Philippines have explicit policies against the payment of ransoms, and the deadline passed. The grisly evidence of Ridsdel's execution was dropped off in a plastic bag on a street in Jolo, capital of Sulu province, a long-standing cradle of Islamist militancy off the southwest coast of Mindanao.

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"Canada condemns without reservation the brutality of the hostage-takers, and this unnecessary death," Canadian Prime Minister Justin Trudeau said in a statement. "This was an act of cold-blooded murder and responsibility rests squarely with the terrorist group who took him hostage."

The government in Manila ordered the country's military and police to aggressively search for and rescue the remaining captives. "The intensified operations are ongoing. We have forces on the ground currently pursuing these bandits," army spokesman Restituto Padilla said Monday.

Although it is a majority-Catholic nation, the Philippines still has a significant Muslim population, accounting for about 5 percent of the country's almost 100 million people. As WorldViews noted earlier, Muslim sultanates held sway through much of the archipelago nation, particularly in the south, until the arrival of the Spanish in the 16th century. The Islam practiced there, like other faiths in Southeast Asia, is suffused with older indigenous beliefs, including the worship of local spirits.

The majority of the country's Muslims are now concentrated in Mindanao and adjacent islands. For decades, various separatist insurgencies have raged in the region; most of the groups are motivated more by political grievances and criminal opportunism than ideology.

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Abu Sayyaf first emerged in the early 1990s as a radical splinter group of the Moro Islamic Liberation Front, a rebel faction that wants greater Muslim autonomy in the southern Philippines. Abu Sayyaf's leadership, which was said to have ties with al-Qaeda, aimed to forge its own Islamic state. The group went about emulating the Middle Eastern terrorist organization.

It kidnapped 20 people from a resort in 2001, including three Americans, one of whom was beheaded. In 2004, Abu Sayyaf carried out the worst terrorist attack in the history of the Philippines, targeting a ferry in Manila Bay, leaving 116 people dead. The following year, its militants carried out bombings across the country.

The group has been largely subdued since Manila launched an intense counterinsurgency effort backed by U.S. Special Operations forces. But it has not been defeated. A clash this month with state security forces led to the deaths of 18 soldiers.

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Various cells hold at least 19 hostages, including tourists, as well as Malaysian and Indonesian workers. Current estimates suggest that there are fewer than 400 fighters in Abu Sayyaf's ranks, a significant dip from its heyday in the 1990s. Moreover, there is no evidence that Abu Sayyaf receives any material support from al-Qaeda.

What has alarmed international observers were the apparent declarations of fealty to the Islamic State militant group made by elements of Abu Sayyaf this year, including one by the group's titular head, Isnilon Hapilon, who has now styled himself Sheik Mujahid Abu Abdullah al-Filipini.

The specter of a transnational terrorist threat is a concern to many countries in the region. Hundreds of Southeast Asian extremists have joined the Islamic State's ranks in Syria and Iraq. And suspected Indonesian proxies of the group carried out a deadly assault in central Jakarta in January.

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On Tuesday, Indonesian President Joko Widodo called for joint maritime patrols with Malaysia and the Philippines to better combat Abu Sayyaf and other militant groups operating in islands and communities far from the reach of all three nations' capitals.

"We can’t let this continue," the Indonesian leader declared.

Some experts believe that the key to quieting the insurgency in the Philippines is extending the rule of law and pushing through a peace process with other militant groups that are already in talks with Manila. Progress has stalled ahead of the country's presidential election, scheduled for May.

"Abu Sayyaf was formerly what we all think of when we think of a terrorist organization," Greg Poling, director of the Asia Maritime Transparency Initiative, told CNN. But times have changed, he adds: "This is basically a group of criminals whose only support comes from family connections in the local communities."