At least 40 people have lost their lives after a fire broke out at a migration center located along the U.S.-Mexico border, according to the country’s National Migration Institute.

The facility, in Ciudad Juárez, is on the Mexico side of a bridge that connects Ciudad Juárez and El Paso, Texas, and is intended to be a short-term processing center for migrants in transit.

Mexican President Andrés Manuel López Obrador said that the fire was caused by a protest initiated by some of the migrants detained at the center, “after, we think, they found out they’d be deported.”

The migrants are believed to have put small mattresses at the door of the shelter and set them on fire “as a form of protest,” López Obrador said at a news conference on Tuesday morning.

The country’s National Migration Institute has not yet revealed the cause of the fire. It said that it “strongly rejects the acts that led to this tragedy,” without elaborating on what they may have been. Dozens more people were injured, with 29 of them in “delicate-serious condition,” according to the agency.

Guatemala’s General Directorate of Migration confirmed that 28 of the dead were Guatemalans. According to the country’s prosecutor general, which has opened an investigation, 13 Hondurans, 12 Salvadorans, 12 Venezuelans, a Colombian and an Ecuadorian were among the 68 people affected by the blaze. Consular teams were also being engaged to further identify the dead, officials said.

Betty Camargo, the state programs director at the Border Network for Human Rights, a human rights advocacy and immigration reform organization in the U.S.-Mexico border, said she has been speaking to migrants who witnessed the fire at the center and the events that preceded it.

Authorities inside the center told some of the detained migrants they were being deported, even though many of them had temporary work permits that are renewed every month, Camargo said. The migrants said they were told such permits would be taken away from them.

Fernando García, executive director of the Border Network for Human Rights, said that such migration centers “should not be detention centers.” Most migration centers run by the National Migration Institute are intended to be processing centers and “shelters” for short stays for migrants in transit. The organization has called for an investigation into the fire.

Ciudad Juárez is a major crossing point for migrants trying to make the journey across the border to the U.S. Its shelters are full of migrants waiting for opportunities to cross or who are waiting out the asylum process. In recent years, as Mexico has stepped up efforts to stem the flow of migration to the U.S. border under pressure from Washington, its National Immigration Institute has struggled with overcrowding in its facilities.