"The attackers entered the bus and shot the passengers in the head," said a police official on condition of anonymity.
The bus in which passengers were traveliing belongs to the Al-Azhar Garden Colony, which is an Ismaili community housing project in the city. It was on its regular route headed towards Federal B Area of Karachi.
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Around 50 to 60 people were travelling on the bus when the attack took place. The attackers managed to escape from the scene after the incident.
Police and Rangers personnel cordoned off the area. No one has claimed responsibility of the attack yet.