CIUDAD JUAREZ, Mexico (AP) — After migrants in northern Mexico placed mattresses against the bars of their detention cell and set them on fire, guards quickly walked away and made no apparent attempt to release the men before smoke filled the room and killed at least 40, surveillance video showed Tuesday.

Hours after the fire broke out late Monday, rows of bodies were laid out under shimmery silver sheets outside the immigration detention facility in Ciudad Juarez, which is across the US border from El Paso, Texas, and a major crossing point for migrants.

Twenty-nine people were injured and were 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.

In the video, two people dressed as guards rush into the camera frame, and at least one migrant appears by the metal gate on the other side. But the guards did not appear to make any effort to open the cell doors and instead ran away as billowing clouds of smoke filled the structure within seconds.

Adán Augusto López, Mexico’s interior secretary, confirmed the authenticity of the video in an interview with local journalist Joaquín López Doriga.

Immigration authorities identified the dead and injured as being from Guatemala, Honduras, El Salvador, Venezuela, Colombia and Ecuador, according to a statement from the Mexican attorney general’s office. Guatemala Foreign Affairs Minister Mario Búcaro said 28 of the dead were Guatemalan citizens.

Mexico 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.

The deaths forced the government to rent refrigerated trailers to hold the migrants’ bodies, Chihuahua state prosecutor Cesar Jáuregui told reporters.

The detention facility is across the street from Juarez’s city hall.