At least 39 migrants from Central and South America died after a fire broke out late on Monday at a migrant detention center in the Mexican northern border city of Ciudad Juarez, apparently caused by a protest over deportations, officials said on Tuesday.

Twenty-eight of the dead at the center were Guatemalans, Guatemala’s national migration institute said.

Mexican President Andres Manuel Lopez Obrador said authorities believed the blaze in the city opposite El Paso, Texas, broke out at 9:30 p.m. local time as some migrants set fire to mattresses in protest after discovering they would be deported.

“They didn’t think that would cause this terrible tragedy,” Lopez Obrador told a news conference, noting that most migrants at the facility were from Central America and Venezuela.

The fire, one of the most lethal to hit the country in years, occurred as the United States and Mexico are battling to cope with record levels of border crossings at their shared frontier.

A Reuters witness at the scene overnight saw bodies laid out on the ground in body bags behind a yellow security cordon, surrounded by emergency vehicles. The fire had been extinguished.

Karla Samayoa, a spokesperson for Guatemala’s foreign ministry, said Mexican officials had informed them that some Venezuelans at the center had set mattresses alight.

All told, there were 68 adult men from Central and South America staying at the facility in the city opposite El Paso, Texas, Mexico’s National Migration Institute (INM) said.

On top of the 39 that died, 29 others were in hospital and in serious condition after being injured in the blaze, the office of the attorney general said.

As migration has risen in recent years, advocates have frequently flagged concerns of poor conditions and overcrowding in the detention centers run by Mexico’s immigration authorities.

“Last night’s events are a horrible example of why organizations have been working to limit or eliminate detention in Mexico,” said Gretchen Kuhner, director of the Mexico-based Institute for Women in Migration that supports migrant rights.