Police say bombing may not be revenge attack by separatist group.
The country has been rattled by a number of explosions in recent weeks.
The congressional building in Manila was bombed last week in an apparent assassination attempt on a Muslim politician.
The attack came barely a month after an explosion in a shopping mall in the capital Manila killed 11 people.
Police have confirmed that the Glorietta mall blast was caused by a gas explosion but a final report on the cause will be completed within the coming weeks.