Abu Sayyaf is a violent extremist group that split from established Philippines separatist movement Moro National Liberation Front in 1991. It was formed by Abdurajak Abubakar Janjalani, who trained in the Middle East and reportedly met with al Qaeda leader Osama bin Laden.
The Philippines is a predominantly Catholic country, but the south has historically had a large Muslim population. Abu Sayyaf's stated aim is to establish an independent Islamic state on the southern island of Mindanao.
The group first became active in the early 1990s and was responsible for bombings across the southern Philippines and in the Malaysian state of Sabah.
Following the 9/11 attacks, Washington worked hard with the Philippine military to try and stop the group's terrorist activities.
Largely weakened by these attacks, Abu Sayyaf started to move from large-scale bombings to kidnappings, a move many analysts see as more profit-driven and criminal in intent.
"Abu Sayyaf was formerly what we all think of when we think of a terrorist organization," said Greg Poling, director of the Asia Maritime Transparency Initiative
at the Washington-based Center for Strategic International Studies.
"But after about 15 years of a pretty harsh crackdown by the U.S. and the Philippines, what they've basically become is a criminal group made up of a few hundred who engage in extortion and kidnapping."
Poling said the group is now largely fragmented, with around 200 to 400 members, and lacks the popular support base it enjoyed in the early 1990s.
"This is basically a group of criminals whose only support comes from family connections in the local communities," he said.
Abu Sayyaf reportedly was trained and funded by al Qaeda and Indonesia-based militant group Jemaah Islamiyah at the outset, but this well-publicized connection to established overseas Islamist extremist groups remains tenuous.
A small number of group members swore allegiance to ISIS
in a video posted to YouTube in 2014, but Poling said ISIS has been "very hesitant to recognize that formerly, to set up a caliphate in Southeast Asia."
Abu Sayyaf was inspired by al Qaeda and has undoubtedly received financial support from overseas extremist groups in the past, but there's no evidence of such support today, Poling said.
"There's still no direct evidence of support or funding from IS (ISIS) of anything beyond the, I guess cache, that Abu Sayyaf, or some members of Abu Sayyaf, felt like they got from this public swearing," he said.
Are there other hostages?
Now with Ridsdel's death, 13 other foreign nationals remain captive by Abu Sayyaf, according to the Armed Forces of the Philippines
They include Canadian Robert Hall, Norwegian Kjartan Sekkingstad and Filipina Marites Flor -- who were kidnapped along with Ridsdel -- as well as a Dutchman, and 10 Indonesian sailors who were abducted in the waters off the southern Philippines
. A number of Filipinos are also being held hostage.
In early April, authorities recovered Italian missionary Rolando del Torchio
in the southern Philippines after Abu Sayyaf had abducted him in October.
Can Abu Sayyaf be defeated?
Despite a concerted campaign by the Philippines military
to halt Abu Sayyaf's activities -- an operation the U.S. and Australian governments have supported -- Poling said it's disappointing the group hasn't been eradicated.
However, he said significant inroads have been made in reducing its membership and many of its leaders have been killed, including the group's founder, Janjalani, in 1998.
A national Philippines court also declared the group a terrorist organization under a rarely used law in 2015, effectively outlawing membership.
However, Poling said obstacles remain in eradicating the group completely, including the weakness of the Philippine military across much of the south, the porousness of the region's borders with Malaysia and Indonesia, and its remote geographic location.
"Once they get into the jungles of Mindanao, there's a saying that the Philippine state controls the roads and others control the countryside down there," he said.
Renewed international attention may help overcome these roadblocks, but it may be of little consolation to those who remain hostages under Abu Sayyaf in the jungles of Mindanao.