At least 40 people were killed when a fire broke out inside a migrant detention center in the Mexican border city of Ciudad Juárez on Monday night (March 27).

Authorities said some of the detainees lit mattresses on fire inside the facility to protest deportations. During a press conference Tuesday (March 28), Mexican President Andrés Manuel López Obrador said they started the fire “as a form of protest … after they found out they’d be deported.”

He added: “They never imagined that this would cause this terrible misfortune.”

According to NBC News, the victims, all men, inside Estancia Provisional de Ciudad Juárez were from Guatemala, Honduras, Venezuela, El Salvador, Colombia, and Ecuador. There were also at least 28 people seriously injured.

Now, Obrador and others are asking why the men weren’t let out of the building before the fire spread. “From the investigation, it is clear that there was negligence, but we still need to know exactly what happened,” Obrador said at another news conference Wednesday morning (March 29). “There will be no impunity.”

Viangly Infante Padrón told NBC News that she was waiting outside the detention center for her husband to be released when the fire began. Her husband, Eduard Caraballo López, survived the tragedy.

“There was smoke everywhere,” Padrón said. “The ones they let out were the women, and those (employees) with immigration. They never took [the men] out until the firefighters arrived.”

She added: “They alone had the key. The responsibility was theirs to open the bar doors and save those lives, regardless of whether there were detainees, regardless of whether they would run away, regardless of everything that happened. They had to save those lives.”

In a video from inside the center, guards are seen not opening the cells where the migrants were housed as smoke filled the facility.