Nigerian authorities suspect the insurgent group Islamic State West Africa Province (ISWAP) carried out a massacre in a Catholic church on Sunday in which 40 people were killed, Interior Minister Ogbeni Rauf Aregbesola said on Thursday.

Assailants wielding AK-47 rifles and explosives attacked the congregation at St Francis Catholic Church in Owo, in southwestern Ondo State, during Pentecost mass on Sunday, leaving behind a scene of carnage as they escaped.

“We have been able to see the footprint of ISWAP in the horrendous attack in Owo and we are after them. Our security agencies are on their trail and we will bring them to justice,” Aregbesola told reporters in the capital Abuja.

The authorities had not previously made any comment about the identity or motive of the killers.

ISWAP, predominantly active in northeastern Nigeria and neighbouring Chad, is one of two major Islamist insurgent groups that have been fighting each other and the Nigerian military for years. Hundreds of thousands have died and millions have been displaced.

Ondo State is far from ISWAP’s usual area of operations.

The state governor, Arakunrin Akeredolu, gave new casualty figures on Thursday. He said a total of 127 people had been affected by the attack in the church, of whom 40 had died, 61 were still in hospital and 26 had been discharged.

Previously, an official from the National Emergency Management Agency had given a death toll of 22. Akeredolu said the new casualty figures had been compiled from reports from multiple hospitals including private ones.

He said the state government would provide land for a mass burial of the victims, but did not say when that would take place.

“We will have a memorial park here where those who died in the attack will be buried,” he said, addressing Catholic clergymen who had come on a condolence visit.