Preliminary reports said the blast could have been caused by a liquefied petroleum gas leak in a restaurant in the mall, however traces of C4 plastic explosive were later found.
"From the destruction that I saw, this wasn't caused by a liquefied petroleum gas leak," the newspaper quotes police chief, General Avelino Razon, as saying. "This was a bomb, but beyond that I cannot say anything, because investigations are still ongoing."
After surveying the site, investigators say the bomb, which went off at lunch time, could have been planted close to a nearby cell phone repair shop. The blast also caused extensive damage to the three-story building of Glorietta shopping mall.
Police blocked off the district, and subsequently issued a general alert covering the city and the international airport. Philippine President Gloria Macapagal Arroyo convened an emergency meeting of the National Security Council and promised that the explosion would be thoroughly investigated.
No one has claimed responsibility for the blast so far.