Manila , Philippines ( CNN ) -- A powerful bomb exploded on a passenger bus Tuesday afternoon in Manila 's financial district , killing four people and injuring 15 others , police said . `` It 's a huge explosion ; it 's so huge that it created a hole at the side of bus where it is presumed that the bomb was placed , '' said Makati Police Chief Froilan Bonifacio . Bonifacio said the explosion occurred around 2 p.m . The bus , owned by Newman Bus Lines , was approaching a Metro Rail Transit station on Buendia Avenue in Makati City , Manila 's financial district , when it blew up . The area has been placed on high alert , according to Makati Mayor Jejomar Binay Jr. , who arrived on the scene a short time after the blast . He said most of the victims were taken to nearby hospitals for treatment . `` We are still trying to determine what kind of bomb was used , '' Bonifacio said . An initial investigation showed the explosive device could have been placed under a seat on the bus , he said . Journalists Arlene Espirtu and Winona Cueva contributed to this report .
A police chief says the bomb might have been planted under a seat . Police are unable to determine the type of bomb . The bomb blew a hole through the side of the seat . Makati City is Manila 's financial district .