MEXICO CITY (AP) — Immigration agents said Monday they came upon a heart-rending scene on a roadside in sweltering southern Mexico — a tractor-trailer with its door open and a migrant who had apparently suffocated to death inside.

Around the truck lay eight other migrants too weak to move. Nearby, a 2-year-old boy stood alone by the guard rail, apparently abandoned by whoever had been accompanying him on the clandestine trip to the U.S. border, authorities said.

The survivors told agents that more than 100 migrants had been crammed into the trailer. When they could no longer bear the lack of air and high temperature inside the closed freight container, they banged on the doors to be released.

The driver or his companion opened the door, and most of the migrants ran off into heavy vegetation on the side of the highway, the survivors said.

But one 25-year-old man was already dead, and eight others collapsed on the pavement or inside the truck. None of the survivors was related to the boy, who is apparently from Guatemala, officials said.

The National Immigration Institute said the boy has been put in the care of child welfare officials until his relatives are found, and the surviving adults were taken to an immigration station.