MEXICO CITY, Mexico (AP) — Migrants fearing deportation set mattresses ablaze at an immigration detention centre in northern Mexico, starting a fire that killed at least 40 people, the president said Tuesday, in one of the deadliest events ever at a Mexican immigration lock-up.

Hours after the fire broke out late Monday, rows of bodies were laid out under shimmery silver sheets outside the facility in Ciudad Juarez, which is across from El Paso, Texas, and a major crossing point for migrants. Ambulances, firefighters and vans from the morgue swarmed the scene.

Twenty-nine people were injured and were in “delicate-serious” condition, according to the National Immigration Institute.

At the time of the blaze 68 men from Central and South America were being held at the facility, the agency said.

Immigration authorities identified the dead and injured as being from Guatemala, Honduras, El Salvador, Venezuela, Colombia and Ecuador, with Guatemalans being the largest contingent, according to a statement from the Mexican attorney general’s office.

Guatemala Foreign Affairs Minister Mario Búcaro said 28 of the dead were Guatemalan citizens.

“We are going to look to find those responsible for this,” Búcaro said.

Mexico President Andrés Manuel López Obrador said the fire was started by migrants in protest after learning they would be deported.

“They never imagined that this would cause this terrible misfortune,” López Obrador said, adding that the director of the country’s immigration agency was on the scene.

The detention facility is a short walk from the US border and across the street from Juarez’s city hall.

Tension between authorities and migrants had apparently been running high in recent weeks in Ciudad Juarez, where shelters are full of people waiting for opportunities to cross into the US or who have requested asylum there and are waiting out the process.

More than 30 migrant shelters and other advocacy organisations published an open letter March 9 that complained of a criminalisation of migrants and asylum seekers in the city. It accused authorities of abusing migrants and using excessive force in rounding them up, including complaints that municipal police questioned people in the street about their immigration status without cause.

The high level of frustration in Ciudad Juarez was evident earlier this month when hundreds of mostly Venezuelan migrants tried to force their way across one of the international bridges to El Paso, acting on false rumours that the United States would allow them to enter the country. US authorities blocked their attempts.

The national immigration agency said Tuesday that it “energetically rejects the actions that led to this tragedy”, without any further explanation.

The “extensive use of immigration detention leads to tragedies like this one,” Felipe González Morales, the United Nations special rapporteur for human rights of migrants, said via Twitter. In keeping with international law, immigration detention should be an exceptional measure and not generalised, he wrote.

As Mexico has stepped up efforts to stem migration to the US border under pressure from the American Government, the agency has struggled with overcrowding in its facilities. The country’s immigration lock-ups have seen protests and riots from time to time.

Mostly Venezuelan migrants rioted inside an immigration centre in Tijuana in October, which had to be controlled by police and National Guard troops. In November dozens of migrants rioted in Mexico’s largest detention centre in the southern city of Tapachula near the border with Guatemala. No one died in either incident.

Mexico has emerged as the world’s third most popular destination for asylum seekers, after the United States and Germany. But it is still largely a country that migrants pass through on their way to the US.