(LONDON) -- Some of the gunmen who attacked a church full of worshippers in southwestern Nigeria on Sunday were disguised as congregants, police said.
Dozens of people, including women and children, were killed in the late-morning attack at St. Francis Catholic Church in the town of Owo in Ondo state, more than 200 miles northwest of Lagos, Nigeria’s largest city, and over 200 miles southwest of Abuja, the Nigerian capital.
More than 80 people are feared to have died, sources told ABC News. Police officials said they are still investigating and working to determine what the final casualty count is.
A source with direct knowledge of the investigation said there were 82 bodies in the local morgue from Sunday’s attack. Another source briefed on the latest U.S. intelligence assessment said the estimate was more than 80.
The church was holding a service for Pentecost Sunday, a Christian holiday celebrated on the 50th day after Easter, when suddenly explosives detonated and gunshots rang out at around 11:30 a.m. local time, according to Olumuyiwa Adejobi, a spokesperson for the Nigeria Police Force's headquarters in Abuja.
"Further investigations revealed that some of the gunmen disguised as congregants, while other armed men who had positioned themselves around the church premises from different directions fired into the church," Adejobi said in a statement on Monday night.
An unknown number of gunmen had approached the church during the service and began shooting at worshippers as they tried to flee, according to Funmilayo Ibukun Odunlami, a spokesperson for the Nigeria Police Force's command in Ondo state, adding that several gunmen were firing from inside the building.
A motive for the massacre was not immediately clear, as no group has claimed responsibility. Police have yet to identify the perpetrators or release the number of casualties.
"Some lives were lost and some sustained varying degrees of injuries," Odunlami said in a statement on Sunday evening, later telling ABC News that police do not yet have an estimate.
Health workers at the Federal Medical Center in Owo told ABC News on Monday that at least 35 bodies had been transported to the hospital from the scene of Sunday's attack. They said there was also an urgent need for blood donations for the wounded.
Meanwhile, the Catholic Laity Council of Nigeria said in a statement on Monday that "more than 50 parishioners" had died and the gunmen were "suspected to be bandits."
The suspects fled the scene in a stolen Nissan and remain at large. The vehicle has since been recovered by police, according to Adejobi.
Police have also recovered three undetonated improvised explosive devices (IEDs) from the scene, along with fragments of detonated IEDs and pellets of expended AK-47 ammunition, Adejobi said.
Nigerian Inspector-General of Police Usman Alkali Baba has ordered a "full-scale" and "comprehensive" investigation into the incident and has deployed specialized police units to help track down the assailants, according to Adejobi.
"He equally assures that the heartless killers of the harmless victims, particularly innocent children, would be made to face the full wrath of the law," Adejobi said.
ABC News' James' Bwala in Abuja, Nigeria, and Josh Margolin contributed to this report.