It was not immediately clear who carried out the attack. But in the past, Sunni militant group Lashkar e Jhangvi has targeted buses of Shiite pilgrims.
Shia Islam is a minority sect in the mainly Sunni Muslim country, and its members face persecution from extremists.
MORE: http://www.cnn.com/2012/12/30/world/asia/pakistan-pilgrims-killed/index.html?hpt=ias_c1
See also...
21 Pakistani security officers believed shot to death by Taliban
http://www.cnn.com/2012/12/29/world/asia/pakistan-taliban-capture/index.html?hpt=ias_c2
