At least 39 people were killed after a fire broke out at a migration center along the U.S.-Mexico border, officials said Tuesday.

The fire began Monday night at a facility run by the National Migration Institute in Ciudad Juarez, the agency said in a statement.

Dozens more were injured, with 29 people taken to four hospitals in “delicate-serious condition,” it said, adding that there were 68 men from Central America and South America being held in the facility at the time of the fire.

The cause of the fire was not immediately known. The agency said that it “strongly rejects the acts that led to this tragedy,” without elaborating on what they may have been.

Authorities were investigating and the government’s National Human Rights Commission had been called in to help the migrants, it said.

Ciudad Juarez is a major crossing point for migrants trying to make the journey across the border to the United States.

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.