Police and a local official in Balad said on Thursday that the attack happened overnight as the pilgrims were returning from a revered Shiite shrine in Samarra.
The officials spoke on condition of anonymity because they were not authorized to speak to the media.
Samarra is the burial site for two revered 9th century Shiite imams and on Wednesday, there was a ceremony marking the death of one of them.
The region north of Baghdad is predominantly Sunni and last year much of it was seized in a blitz by the extremist Islamic State group.
