Men then marched to the front of church, near the altar, and opened fire as worshipers scattered. Some of the gunmen blocked an exit door.
Nigeria, Africa’s most populous nation, faces challenges on multiple fronts, including the Islamist insurgency Boko Haram in the north and a militant faction called the Niger Delta Avengers in the oil-rich south.
Among the 12 dead in Ozubulu was the father of Aloysius Ikegwuonu, 36, a wealthy local magnate who had helped build the church.
Witnesses said that the gunmen been searching for Mr. Ikegwuonu, who lives in the area, and that they had gone to the church to find him.
Credit Mike Agada/European Pressphoto Agency
Drug dealers exert a powerful hold on Ozubulu and other towns in Anambra State. They are often generous donors to local political and religious figures. Some observers say that they also pay bribes to the security forces, which then allow them to do their work unmolested.
Gang-related violence is not uncommon in towns like Ozubulu, where drug cartels are active; one even occurred recently during the funeral of a reputed gang member who had died in South Africa.
The governor, Mr. Obiano, declared Monday a day of mourning, and in the afternoon, Bishop Okeke celebrated Mass in honor of the victims.
Mr. Obiano said he had gotten phone calls from Nigeria’s president, Muhammadu Buhari, who is in London for medical treatment, and from Vice President Yemi Osinbajo, who is acting as president.
The church killings have produced an outpouring of denunciations across the country.
“Whatever may have been the situation, this attack represents a descent into the pit of hell,” the Nigerian Labour Congress said in a statement from Abuja, the capital. “It is animalistic, cowardly and shameful.”
However, others said that religious leaders needed to do more to make clear that they shun violence, even if it meant declining donations if they were not certain of the source of the money.
Some religious leaders “worship money irrespective of where it comes from and have now become part of the alliance of evil,” said Adeola Soetan, an activist.
Mr. Ikegwuonu, the local magnate, did not respond to questions asking about the source of his wealth.
Bishop Okeke did not respond to messages requesting comment about Mr. Ikegwuonu and his role in financing the construction of the church.
The Rev. Hyginus Aghaulo, director of communications for the Diocese of Nnewi, said in a statement: “The prophetic church’s mandate compels her to love the sinner and hate his evil ways. The church does not say: You are already in hell and have nothing to do with us. No, she uses every available opportunity to continue to call sinners to repentance.”