MANILA - A deadly bomb on a Manila bus appears similar to devices used recently on Mindanao island, a troubled region where Muslim separatists and Maoists guerrillas are active, the Philippine government said on Wednesday.
No group has claimed responsibility for Tuesday's blast.
The death toll rose to five after a woman died in hospital overnight. Thirteen others were injured when the bomb exploded on the bus as it was travelling on the city's main highway near the central business district.
Interior Secretary Jesse Robredo said preliminary findings showed the bomb used an 81 mm mortar with a timing device. Police have said the device, which blew a hole in the side of the bus, was placed under a seat about halfway down the vehicle.
"From our experience, this type of bomb was used in Mindanao in recent months. In terms of the motive, we cannot determine yet... but we can say the universe of possible groups is now a little bit smaller," he said in a radio interview.
"This means whoever did this has knowledge, this is not some ordinary person who just thought of doing damage, this means the person responsible has had experience, the person has had training."
Last year, security forces said a splinter group of Muslim separatists was behind a bus attack on Mindanao that killed 10 people. There have also been bombings in the south related to extortion, as well as political and clan disputes.
In November, a number of foreign embassies had updated travel advisories warning of a heightened risk of attack, including in Manila, which the government had said was not justified.
The Philippines has been trying to end two long-running insurgencies by Muslim separatists and Maoist rebels. Peace talks with both groups are due to restart next month.
Islamic militants linked to the Abu Sayyaf and Jemmaah Islamiah groups operate on remote southern islands, although they have been largely contained in recent years, with assistance from U.S. military advisors.
President Benigno Aquino said on Tuesday there had been intelligence last year that a Muslim secessionist group had been planning attacks, but they were judged not to have the capability. Those assessments were being reviewed.
"So that is what I have asked to be re-evaluated," Aquino said. "Is it the same threat group, are they different threat groups, who is responsible?"