At least two people have been shot dead and 11 wounded after gunmen launched an overnight attack on a church in northern Nigeria, officials say.The attack happened in the Kaduna state town of Zonkwa, which saw serious violence after April's election.Kaduna is divided along political, ethnic and religious lines and the BBC's Nura Ringim in the state says it is thought to be a revenge attack.Thousands of Muslim Hausas and Fulanis were forced from their homes in April.Our correspondent says that some of those attacked had vowed to take revenge, as had the Boko Haram Islamist militant group, based further north in Borno state.He says some 3,000 people are still living in a camp in the state capital, Kaduna, after their houses were burnt in the Zonkwa post-election clashes, which left several people dead.A police spokesman told our correspondent that some of those shot in the latest attack on a congregation performing a night vigil were in a critical condition.Kaduna mirrors Nigeria as a whole, with the south largely inhabited by Christian groups, while Muslims form a majority in the north.In April's elections, Patrick Ibrahim Yokowa became the state's first Christian governor.He belongs to the People's Democratic Party, which governs at the federal level.The opposition Congress for Progressive Change (CPC) gains much of its support from Muslim groups.More than 1,000 people have been killed in recent years in neighbouring Plateau state in a deadly spiral of revenge attacks between rival groups similarly split along ethnic, religious and political lines.