A mass shooting at the St. Francis Catholic Church in southwestern Nigeria claimed the lives of at least 28 people, CNN reports.

According to the report, four assailants stormed the church in the city of Owo during mass and began “shooting sporadically,” says Adeyemi Olayemi, a legislator in the Ondo State House of Assembly.

“The attackers came in motorcycles and started shooting sporadically,” he said. “They killed many people inside the church.”

The attack on Pentecost Sunday was the deadliest attack on a church in Nigeria in years.

President Muhammadu Buhari, who has promised to bring stability to Nigeria, condemned the fatal attack as a “dastardly act.”

In a post on Twitter, Ondo State Governor Rotimi Akeredolu said he was “shocked” by the attack and called it a “black Sunday in Owo.” He vowed to “commit every available resource to hunt down these assailants and make them pay.”

“I am deeply saddened by the unprovoked attack and killing of innocent people of Owo, worshiping at the St. Francis Catholic Church, today,” his tweet said, adding that “the vile and satanic attack is a calculated assault on the peace-loving people of the Owo Kingdom who have enjoyed relative peace over the years.”

“We shall never bow to the machinations of heartless elements in our resolves to rid our state of criminals,” he continued.

“I urge our people to remain calm and vigilant,” he added. “Do not take laws into your hands. I have spoken to the heads of the security agencies. I have equally been assured that security operatives would be deployed to monitor and restore normalcy to Owo kingdom.”

The Owo church massacre follows another horrific church mass shooting in Nigeria, when 31 people were killed and several were injured during a stampede at a church event in the city of Port Harcourt in southeastern Nigeria.