Security officials said in Manila yesterday that a powerful bomb that tore through a popular night market in President Rodrigo Duterte's home city of Davao was set off with a mobile phone, and that they are now searching for three "persons of interest".
The blast killed at least 14 people, and led Mr Duterte to declare a "state of lawless violence" covering all of the Philippines.
Senior Superintendent Michael John Dubria, police chief of Davao, said in a news briefing the "highly explosive arsenal" could have been placed inside a massage booth at around 10.30pm supposedly by a man seen fleeing the scene just before the explosion.
Police are seeking two other "persons of interest", both women.
Senior Supt Dubria said "there may be more persons of interest".
He said investigators are already talking to eight witnesses, and reviewing footage from eight closed-circuit TV cameras.
Senior police officer Consorcio Gerones Jr from Davao's bomb squad, said the bomb was made from parts of a 60mm mortar.
A piece of shrapnel that formed the upper portion of the fuse assembly was retrieved from the liver of one of the injured victims, he said.
Abu Sayyaf, a small but violent militant group linked to the Islamic State in Iraq and Syria, is widely believed to be behind the attack.
Mr Duterte, who was Davao mayor for nearly two decades before becoming president in July, said the attack could have been in retaliation for an ongoing major assault on the Abu Sayyaf's stronghold of Jolo island, about 900km from Davao. Fifteen soldiers and at least 21 extremists have died in clashes in Jolo.
Some 7,000 soldiers are now pursuing the Abu Sayyaf.
Malaysian security forces, meanwhile, are on high alert off the coast of Sabah for militants fleeing the fighting in Jolo, closely monitoring sea routes, landing points and other hot spots,
Senior Supt Dubria said police were also looking at the possibility that the blast could have been the work of "lawless armed groups or illegal drug syndicates".
"We have threats from the drug groups... those affected by our illegal drug operations," he said.
Mr Duterte has pursued a brutal anti-crime war since he took office on June 30. Over 2,000 drug suspects, including purported kingpins, have been killed by police and vigilante groups.
Police believe these killings have driven syndicates out of the streets, but certain "drug lords" have begun plotting to have Mr Duterte assassinated.