At least eight people are now known to have been killed in Tuesday's bomb explosion in the southern Philippines.
The bomb went off in a billiards hall in Tacurong
Military officials have blamed the militant Islamic group Jemaah Islamiah for the attack on Mindanao island.
But analysts suggest national politics may also be to blame. Nearly 100 people have been killed in violence relating to next week's parliamentary elections.
Police and soldiers have stepped up security throughout the country because of the upcoming polls.
Nearly 87,000 candidates are competing for 17,000 national and local positions.
All 265 House of Representatives seats and half of the 24 Senate seats are also being contested.
Tuesday's explosion happened in the city of Tacurong, on Mindanao island, some 950km (600 miles) south of Manila.
The bomb ripped through a billiards hall in a market area of the city, killing at least three people on the spot. Five more died overnight.
The security forces pointed the finger at the regional militant group Jemaah Islamiah (JI) as the possible perpetrators of the attack.
"We have strong suspicions the JI carried out the attack to remind us they're still very much around," Major-General Raymundo Ferrer told reporters.
"The target was obviously innocent people".
Mindanao has been plagued for more than three decades by an ongoing Muslim insurgency.
Officials ruled out any link with politics, but the attack comes as the country prepares to go to the polls on 14 May.
The Philippines has been wracked by pre-election violence in recent months - not unusual in a country where rival politicians often maintain their own armed groups to harass and intimidate opponents.
A village official and the driver of another candidate became the latest victims of the violence on Wednesday, when they were killed in separate attacks in the volatile northern province of Nueva Ecija, the Associated Press reports.
The military announced that the country's forces had been put on nationwide red alert and all leave had been cancelled ahead of the elections.
"Let's say the whole armed forces is on call. Some areas have been declared as election areas of concern so we are anticipating more deployments," Armed Forces chief Hermogenes Esperon Jr said.