Manila - A Malaysian man held by Islamic militants for six months in the southern Philippines was confirmed killed on Wednesday.
Here are a series of questions and answers on the Abu Sayyaf, a US-listed terrorist organisation blamed for the murder of Bernard Then.
The Abu Sayyaf emerged in the early 1990s as a radical offshoot of a Muslim insurgency that has claimed 120,000 lives in the country's south since the 1970s.
From its remote jungle bases on Jolo and Basilan islands the loosely organised group roams on fast boats to snatch hostages, including Western tourists and Christian missionaries, from other parts of the southern Philippines.
The Abu Sayyaf sells itself as the saviour of Muslim communities in Mindanao and leaders last year pledged allegiance to the Islamic State group, which controls vast swathes of Syria and Iraq.
The United States and United Nations list it as a terrorist organisation.
The Philippine military and security analysts regard its members as having little ideology, and as being primarily concerned in lucrative kidnappings for ransom.
From 2002-2014 about 500 US special forces advisers trained and provided intelligence to Philippine troops, which led to the killing or arrest of many Abu Sayyaf leaders.
US assistance was scaled back last year, with American officials saying the Abu Sayyaf's ability to carry out international attacks had been neutralised.
The number of gunmen had been brought down from about 1,000 to roughly 300, according to Philippine military officials.
However, the surviving militants are able to roam the jungles of Basilan and Jolo islands, counting on support from local Muslim communities.
They also entice new recruits, and buy top-grade weapons, with money earned from kidnappings for ransom and other crimes, such as drug trafficking.
Many Western and other embassies routinely issue warnings to their citizens in the Philippines against travel to most of the country's Muslim-populated southern regions because of the risk of being abducted by the Abu Sayyaf.
Recent kidnappings, including of two Canadian tourists and a Norwegian resort owner in areas previously considered beyond the group's reach, has raised fears further.
Then, the murdered Malaysian was kidnapped from a seaside restaurant in Malaysia's Sabah state, about 300 kilometres from the Abu Sayyaf's Jolo stronghold.
The group in 2004 also firebombed a ferry on Manila Bay, more than 1,000 kilometres to the north, killing more than 100 people in what is regarded as the Philippines' deadliest terrorist attack.
President Benigno Aquino's government last year signed a peace deal with the Moro Islamic Liberation Front (MILF), the country's largest guerrilla group, aimed at ending decades of insurgency that the rebels warned has dangerously radicalised young Muslims.
If the deal goes through, the government hopes some of the MILF will be able to contain the much harder-line but smaller Abu Sayyaf.
But, due to a range of political obstacles, hopes are fading that the peace process will be finalised before Aquino steps down next year.