Migrants fearing deportation set mattresses ablaze at an immigration detention center 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 lockup.

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 are 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.

Many of them may have been from Guatemala, according to a Guatemalan official.

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 U.S. border and across the street from Juarez's city hall.

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 organizations published an open letter March 9 that complained of a criminalization 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 rumors that the United States would allow them to enter the country. U.S. 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 generalized, he wrote.

As Mexico has stepped up efforts to stem migration to the U.S. border under pressure from the American government, the agency has struggled with overcrowding in its facilities. The country's immigration lockups have seen protests and riots from time to time.

Mostly Venezuelan migrants rioted inside an immigration center 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 center 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 U.S.

It holds tens of thousands of migrants in an expansive network of detention centers and attempts to closely monitor movements across the country in cooperation with American authorities.

Asylum-seekers must stay in the state where they apply in Mexico, resulting in large numbers being holed up near the country's southern border with Guatemala. Tens of thousands are also in border cities with the U.S., including Ciudad Juarez.

An estimated 2,200 people are in Ciudad Juarez's shelters, along with more migrants outside shelters who come from Venezuela, Nicaragua, Colombia, Guatemala, Ecuador, Peru and El Salvador, according to the Strauss Center for International Security and Law at the University of Texas at Austin.