Migrants were locked in a cell as a blaze spread killing 39 people at a detention center in Mexico, witnesses and a survivor said on Wednesday, as Mexican prosecutors said they were investigating the incident as a possible homicide.

Prosecutors have identified eight people who may be responsible for the deaths on Monday at the center in the northern border city of Ciudad Juarez, Security Minister Rosa Icela Rodriguez said at a news conference Wednesday.

The people are two federal agents, a state migration officer and five members of a private security firm, she added.

All the victims were male, and Mexico’s government is under pressure to find out why they died after officials said the women migrants at the center were safely evacuated.

A short video circulating on social media on Tuesday – appearing to be security footage from inside the center during the blaze – showed men kicking on the bars of a locked door as their cell filled with smoke.

Three uniformed people can be seen walking past without trying to open the door.

“Who didn’t let these people out? Clearly there is a serious crime,” Rodriguez said, noting the video was part of the investigation. “They weren’t capable of opening a gate.”

Emergency protocols and whether the private security company was properly trained, would be examined, Rodriguez added.

“It looks like these guards didn’t have any training,” she said.

Sara Irene Herrerias, head of the human rights unit at Mexico’s Attorney General’s office, said at the same news conference that no arrests have yet been made, but that warrants would be requested this evening and Thursday.

She added the probe would include examining whether a key was available to open the men’s cell, or if there was another way to break the lock.

A man who survived the fire, a paramedic and a security official in the government of Chihuahua state, where Ciudad Juarez is located, all said the cell door where the men were being held remained locked as the fire and smoke spread.

It was not until rescuers arrived that the door was opened, they said. It remains unclear how much time passed between the fire being set and the door being opened.

The survivor, who was being fed oxygen through his nose for smoke inhalation and asked not to be named out of fear of repercussions from migration authorities, confirmed the information in a voice message sent from his hospital bed.

Authorities believe the fire, which killed mostly men from Guatemala and other Central American countries, was started by migrants setting alight mattresses in an act of protest when they discovered they would be deported.

Lopez Obrador said the fire began around 9:30 p.m.

Rescue Team Ciudad Juarez, a private paramedic service, reached the building at 10:05 p.m. and found men in military attire pulling people out of the men’s unit, said a member of the group who was on site and declined to be named.

Outside a hospital in Ciudad Juarez, which sits across the border from El Paso, Texas, family members anxiously waited for news of their loved ones who had been injured in the fire.

Migration officials on Wednesday increased the death toll to 39, saying one person had died of their injuries.

The fire, one of the deadliest migrant tragedies in years, occurred as the U.S. and Mexico are battling to cope with record levels of border crossings at their shared frontier.