Around 8 p.m. local time, a concrete slab was blasted off the building, Two Serendra, sending bystanders in the high-end shopping district into a panic, GMA News reported.
The concrete hit a delivery van, killing three workers. Another vehicle crashed into the van when it swerved after the accident, but the passengers in the second vehicle were unharmed.
Four residents were injured and taken to hospital for treatment.
Initial reports suggested the explosion may have been caused by an appliance in apartment 501, but investigators urged caution before drawing conclusions about the source of the blast.
President Aquino ordered an "unassailable" investigation, directing Philippine National Police, Bureau of Fire Protection, Department of Public Works and Highways, and the Ayala Company to coordinate.
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