A fire broke out at an immigration detention center in Ciudad Juarez, Mexico, late on Monday killing at least 39 people and injuring 29 others. Many of the victims were Venezuelan migrants and asylum seekers. Authorities are investigating the cause of the blaze.

President Andrés Manuel López Obrador of Mexico said in a morning press conference that the fire was the result of a protest by people being held in the facility. Apparently, 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.

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The fire started at about 10 p.m. on Monday evening at the Instituto Nacional de Migración, Mexico’s Institute of Migration said. The facility housed sixty-eight men from Central and South America at the time of the fire, officials said.

Victims were transferred to four local hospitals, authorities said. Those who died included migrants from Guatemala and Honduras, a Mexican official told Reuters.

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The origin of the fire is under investigation. It has presently been extinguished. “I was here since one in the afternoon waiting for the father of my children, and when 10 p.m. rolled around, smoke started coming out from everywhere,” said 31-year-old Viangly Infante, a Venezuelan national.

In recent weeks, there has been a buildup of migrants in Mexican border cities. Authorities are attempting to process asylum requests using a new U.S. government app known as CBP One. According to many migrants, the process has become an extensive one. Earlier this month hundreds of mostly Venezuelan migrants got into a scuffle with U.S. officials at the border as they vented their frustration at the delays the process is causing in the system.