Incoming President Muhammadu Buhari’s pledge to defeat Boko Haram is facing an unexpected adversary: deadly violence between Christians and Muslims that has spread along with the Islamist onslaught in Nigeria’s impoverished northeast.

Saleh Gibril says he had no quarrel with Christians before Boko Haram attacked his hometown of Gulak last year. He fled and told his wife he would see her soon, The Wall Street Journal reported on Saturday.

Instead, he says, all he saw was her muddied hijab, delivered by a friend who took it from her body after Christians who accused her of harboring Islamist militants drowned her in a river.

“They think Boko Haram and Muslims are the same. So they killed her,” says Gibril, a 30-year-old civil servant. Hadiza Ibrahim, who fled with Gibril’s wife, corroborated his story and said she posed as a Christian after seeing what happened to her friend.

There was no way to confirm Gibril’s story or his wife’s innocence. The region where he says his wife was killed is still volatile and all but closed to outsiders. Ibrahim says Christians believed she was one of them, and last month she braved the trip back to battle-scarred Gulak.

Some Christians from the region acknowledge that their brethren have lashed out at Muslims they suspect of colluding with Boko Haram. But they don’t think the sporadic attacks will spread.

Buhari’s pledge to wipe out Islamist insurgency helped propel him to a historic victory last month over the party that long ruled Africa’s most populous country.

A Muslim, he drew support not only from his native and predominantly Muslim north but also from the largely Christian south, an unprecedented rebuke to President Goodluck Jonathan, a southern Christian.