At least 20 people were killed as gunmen opened fire on Muslim worshippers coming out of a mosque in northern Nigeria's Kaduna state on Sunday.
The attack happened in a remote village called Dogo Dawa. A witness said the attack was most likely carried out by a local criminal gang.
"Recently, the local vigilante group had attacked some hoodlums in an area of the state killing some in the process. It is speculated that this could be a reprisal by the group, he said.
Religiously divided Kaduna has seen waves of sectarian violence in recent months.
Suicide bombings at three churches in June that were claimed by Islamist group Boko Haram sparked reprisal violence by Christian mobs who killed dozens of their Muslim neighbours.
Boko Haram wants to install Islamic rule in the country. It has succeeded in killing more than 1000 persons since it started its operations in 2009.
The oil-rich African country has 150 million people who are mostly Christians and Muslims.