A new report has challenged the official version of events during a fire in a Mexican migrant detention facility that killed dozens, alleging that staff could have let the men out of their cell, but instead decided – or were told – not to.

The fire in Ciudad Juárez broke out on 27 March 2023, when detainees started a fire to protest conditions at the facility. But as the flames spread, the men were left in a locked cell as smoke filled the building, until firemen arrived. Forty men were killed, and another 27 survived, with life-altering injuries.

The version of events provided by the National Migration Institute – and repeated by the president, Andrés Manuel López Obrador – has been that the cell could not be opened because staff at the facility did not have the key at the time.

But the new investigation from Lighthouse Reports, in collaboration with La Verdad and El Paso Matters, asserts that the key never left the facility.

Using CCTV from inside the centre, along with other documents, investigators constructed a 3D model of the detention centre and what happened in those minutes.

The investigators say the footage allowed them to track the keys throughout the day. They report that the key to the door of the men’s cell was last seen a minute before the fire broke out, when a security guard gave it to a colleague, who placed it in his pocket.

While the majority of the footage has no sound, the investigators found two CCTV cameras near the main entrance with audio.

According to the new investigation, at one point, a woman in a National Migration Institute uniform can be heard saying, “We are not going to open [the cell] for them, I already told those guys.”

She can be seen talking and texting on her phone, though it is unclear who she was communicating with.

The investigators also found that nearly every fire extinguisher was missing, in the wrong place or defective, that none of the smoke detectors worked, and that the men’s area was both overcrowded and under-ventilated.

The National Migration Institute did not respond to questions about the incident put to them by the investigators.

“[The investigation] is dramatic,” said Tonatiuh Guillén, who led the National Migration Institute during 2018-19. “It confronts the official version of events – of the crime, in truth – that took place in Ciudad Juárez.”

Almost a year has passed since the fire. The incident remains under investigation, with nine of the 11 people charged in the case in pre-trial detention.

These include federal agents, state immigration agents, members of a private security company that was employed in the detention facility, and two migrants accused of starting the fire.

Francisco Garduño Yáñez, the commissioner of the National Migration Institute at the time of the fire, remains in his position, despite being charged with criminal conduct for failure to perform his duties.

The tragedy has highlighted the need to improve conditions in detention facilities, not least because the official number of detentions has soared in recent years.

Compared with before the pandemic, detentions of migrants in an irregular situation have risen fourfold, reaching almost 800,000 in 2023.

Last month, a report from Mexico’s National Commission of Human Rights (CNDH) likened the country’s 49 migration detention facilities to “penitentiary centres” in which people suffer violence and abuse, with inadequate food, poor hygiene and overcrowded cells.

The report recommended a shift towards administrative processes that do not involve detention.

But Guillén says there is pressure, deriving from Mexico’s relationship with the United States, to go the other way, further hardening migration policy.

“We’ve seen this in the militarisation of the [National Migration Institute], as well as the use of the army and the national guard in migration control,” said Guillén. “The proposal from the CNDH is in open friction with the vision of Mexico as a space of contention and dissuasion.”