The bus – which had been taking passengers from Rawalpindi to the northern city of Gilgit – was stopped, before the gunmen ordered the passengers off the vehicle.
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Survivors said the gunmen then opened fire, regional police chief Khalid Omarzai told AFP by telephone.
An unnamed senior police figure told Reuters that all the passengers aboard the bus were Shiite, adding that “it looks like they were targeted by armed men from the local Sunni community."
No group immediately claimed responsibility for the ambush, about 130 miles north of the capital, Islamabad.
Pakistan’s Shiite community accounts for about 15 percent of the country’s population of 180 million.