Six people also were wounded when the gunmen on a motorbike sprayed a minibus with bullets on Wednesday in Quetta, the capital of Baluchistan province.
Police official Sultan Mohammed Gichki says the attack was likely carried out by Wahhabi extremists.
No group has claimed responsibility so far.
Pakistan is a majority Sunni state. Although most Sunnis and Shiites in Pakistan live together peacefully, but Wahhabi extremists often target Shiite leaders and activists.
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