The beheading of a Canadian in the Philippines has sparked fears for the safety of more than 20 other hostages still being held by Islamic militants.
"The Canadian was collateral damage. He was executed to deliver a message," a senior officer working with the Philippine army's special operations forces on Jolo island told The Straits Times.
The severed head of Mr John Ridsdel was found inside a plastic bag thrown in front of a town hall in Jolo, about 1,000km south of the capital Manila, on Monday night.
The 68-year-old former mining executive and journalist from Calgary was beheaded around 3pm on Monday, just as his abductors - believed to be members of the brutal Abu Sayyaf group - had promised if they were not paid 900 million pesos (S$26 million) in ransom.
Canadian Prime Minister Justin Trudeau, who condemned the "heinous killing" of Mr Ridsdel, said his country was working with the Philippines to pursue the killers and free the other hostages.
Abu Sayyaf: Kidnap of foreigners
Twenty-one people were snatched from a resort off Malaysian Borneo. They were freed in 2001.
Three Americans were taken from a marina in Palawan province. One of them, Mr Guillermo Sobero, was beheaded, the first Westerner to be executed.
A former Australian soldier was abducted from his southern Philippine home. He was freed two years later.
A Dutchman, abducted in Tawi-Tawi, is still being held.
Abu Sayyaf gunmen took two Germans from their yacht. They were releasedlater that year.
Two Malaysians were seized from a resort in Sandakan. One was later beheaded, despite payment of ransom.
Canadians John Ridsdel and Robert Hall and Norwegian Kjartan Sekkingstad were taken from yachts docked at a resort on Samal island. Mr Ridsdel was beheaded this week.
A retired Italian priest was snatched from his pizza restaurant in the Philippines. He was released this month.
Four Malaysians and 14 Indonesians were seized from tugboats off Sulu's waters.
Philippine President Benigno Aquino yesterday said security forces would "apply the full force of the law… to neutralise these lawless elements, and bring these criminals to justice".
Mr Ridsdel was abducted last Sept 21 from a resort in Davao del Norte province, in the strife-torn southern Philippines. He was taken with a fellow Canadian, Mr Robert Hall, 50, Norwegian resort manager Kjartan Sekkingstad, 56, and a Filipina, Ms Marites Flor, 40.
A few weeks ago, a video clip showed the three men, with machetes held to their necks, begging for their lives and asking their governments to pay 300 million pesos for each of them, or one of them would be killed at 3pm on Monday.
Efforts to hunt down the heavily armed groups of Abu Sayyaf bandits with a network of checkpoints and battalions of soldiers have proven to be ineffective - and even fatal for both troops and hostages.
On April 9, Abu Sayyaf gunmen killed 18 Filipino soldiers searching for hostages in a day-long battle.
The only operation that had some result was in 2002, when US- backed Philippine special forces ambushed bandits holding American missionaries Martin and Gracia Burnham, and a Filipino nurse, Ms Ediborah Yap. Mrs Burnham was rescued, but her husband and Ms Yap were killed in the crossfire.
One of Abu Sayyaf's biggest recent windfalls is said to have come in 2014, when it claimed to have been paid more than US$5 million (S$6.7 million) for the release of German physician Stefan Okonek, 71, and his companion, Ms Henrike Dielen, 55, who were taken off their yacht. Mr Ridsdel is the third foreigner to be beheaded by the Abu Sayyaf, after Mr Guillermo Sobero, a Peruvian-American, in 2001, and Malaysian Bernard Then last year.
The Abu Sayyaf is believed to be holding more than 20 hostages, including Dutch bird-watcher Ewold Hurn, who was kidnapped in 2012.
During the past month, the group has snatched 14 Indonesians and four Malaysians from tugboats in waters near their strongholds in the Philippines' deep south.