Two of passengers died in hospital on Saturday morning from injuries sustained in the explosion, which has been blamed by the Sri Lankan government and military on the Liberation Tigers of Tamil Eelam (LTTE) separatist group.
The bomb was planted on an overhead rack three rows behind the driver's seat on the bus travelling from the Piliyandala terminus, according to the country's official Media Center for National Security.
The high position of the device caused the vehicle's roof to blow off and inflicted horrific injuries on those travelling in the bus, many of whom were children on their way home from school.
The attack comes amid a fierce conflict in the north of the country between security forces and Tamil Tiger militants, who have been fighting for an independent state for more than 25 years.
