MEXICO CITY (AP) — Migrants fearing deportation set mattresses ablaze at an immigration detention centre in northern Mexico, starting a fire that left 39 dead, the president said Tuesday following one of the deadliest incidents ever at an immigration lockup in the country.

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.

Thirty-nine people died and 29 were injured and are in “delicate-serious” condition, according to the National Immigration Institute. There were 68 men from Central and South America held in the facility at the time of the fire, the agency said. A Guatemalan official said many may have been from that Central American country.

President Andrés Manuel López Obrador said the fire was started by migrants inside the facility 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.

Tensions 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 U.S. 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 the criminalisation of migrants and asylum seekers in the city. It accused authorities of abuse and using excessive force in rounding up migrants, complaining that municipal police were questioning people in the street about their immigration status without cause.

The national immigration agency said Tuesday that it “energetically rejects the actions that led to this tragedy” without any further explanation of what those actions might have been.

In recent years, as Mexico has stepped up efforts to stem the flow of migration to the U.S. border under pressure from the American government, the agency has struggled with overcrowding in its facilities. And the country’s immigration lockups have seen protests and riots from time to time.

Mostly Venezuelan migrants rioted inside an immigration centre in Tijuana in October that 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.

Karla Samayoa, the spokeswoman for Guatemala’s Foreign Ministry, said that Mexican authorities had informed them that more than two dozen of the migrants who died appeared to be from the country.