Philippine authorities on Saturday said the Abu Sayyaf, a local Islamic militant group notorious for kidnapping foreigners, was responsible for a deadly night market bombing that killed at least 14 people.
Here are five key questions and answers about the Abu Sayyaf:
WHO ARE THEY?
The group is a radical offshoot of a Muslim separatist insurgency that has claimed more than 120,000 lives in the south of the mainly Catholic Philippines since the 1970s.
It was established in the 1990s with funds from a relative of former Al-Qaeda leader Osama bin Laden.
Several Abu Sayyaf units have pledged allegiance to the Islamic State group that holds vast swathes of Iraq and Syria but analysts say they are more interested in funding than ideology.
HOW DANGEROUS ARE THEY?
The Abu Sayyaf is blamed for deadly bombings, including an attack on a ferry in Manila Bay in 2014 that claimed 116 lives in the country's deadliest terror assault.
It is also notorious for kidnappings for ransom, murdering foreign and local hostages if huge sums are not paid.
The Abu Sayyaf beheaded an American man in 2002, a Malaysian last year, and two Canadians in April and June.
The United States lists the group as a "foreign terrorist organisation".
The military estimates its forces to number 400, down from an original 1,000 fighters.
WHERE ARE THEY BASED?
Abu Sayyaf's strongholds are the Muslim-populated islands of Jolo and Basilan in the far south of the Philippines, about 1,000 kilometres (600 miles) from Manila.
Sallying forth in fast boats from the islands, the Abu Sayyaf snatches victims and hides them among sympathetic Muslim communities, many of whom have received money from the militants.
In recent months, the group expanded its activities to include high seas kidnappings of sailors in waters bordering the Philippines, Malaysia and Indonesia.
WHAT HAS BEEN DONE TO DEFEAT THEM?
From 2002-2014, the US deployed Special Forces advisers to train and provide intelligence to Filipino troops but scaled back after the Pentagon concluded the group had lost the ability to launch international attacks.
Several Philippine presidents have declared wars on the Abu Sayyaf.
The Philippines, Indonesia and Malaysia also agreed on joint patrols to prevent kidnappings at sea.
But millions of dollars in ransom money, assistance from locals, and their mastery of the terrain have helped the Abu Sayyaf evade government pursuit.
WHY DO AUTHORITIES BELIEVE THEY CARRIED OUT FRIDAY'S ATTACK?
Officials said the Abu Sayyaf was likely to carry out Friday's bombing in retaliation for a military offensive Philippine president Rodrigo Duterte declared last week to "destroy" the group.
National Defence Secretary Delfin Lorenzana said the Abu Sayyaf has a history of launching attacks outside Sulu when pinned down in intense government operations.
The Abu Sayyaf claimed responsibility for three bomb attacks in 2005 -- one in Davao, one in a nearby city and a third in Manila -- that killed eight people.
The Abu Sayyaf said it conducted the 2005 attacks in response to an offensive against it at that time.