The army killed 347 Shiite Muslims during a December clash, according to a special investigation by a state government in northern Nigeria, underscoring deepening animosity between the Shiites and soldiers in a nation already dealing with Sunni Islamist insurgency Boko Haram.

A report released late Monday by the government of Kaduna state, where the killings occurred, found that the army fired at followers of Shiite cleric Sheik Ibrahim Zakzaky, who is currently detained. Mr. Zakzaky’s followers had taken over a highway, blocking a military convoy; and attacked police with improvised explosives, the report added.

Spokespeople for the military and government declined to comment on Tuesday. The military has previously said it was acting in self defense. The protesters “refused all entreaties to disperse,” it said in December.

The cleric’s followers maintain they were parading peacefully.

“There are families who lost children,” said Ibrahim Musa,spokesman for Mr. Zakzaky’s Islamic Movement in Nigeria on Tuesday. “The government is trying to justify the unjustifiable.”

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