A US attorney announced a federal indictment on Friday against 11 people in connection with the smuggling deaths of six migrants whose bodies were found in a rail cargo container in Laredo, Texas, in May. The victims, from Honduras and Mexico, included a 14-year-old boy and ranged up to age 56. They were part of a human-smuggling effort on a freight train, authorities said.

The bodies were discovered by a Union Pacific employee, and the Webb County medical examiner suspected the cause of death was hyperthermia, or heatstroke. “It’s very hard to survive in an un-air-conditioned, sealed container with … the temperature being what it was on that day,” said Justin Simmons, the US attorney for the western district of Texas.

Smuggling on trains has long been a concern, as trains headed to the US often slow or stop in Mexico before crossing the border, providing opportunities for smugglers or immigrants to climb onboard or hide contraband. Paul Nixon, a retired teacher and volunteer with the humanitarian group Green Valley-Sahuarita Samaritans, noted that people are driven to such risks by brutal conditions on foot and cartel abuse. “But people will get into a boxcar and somebody will close the door and just forget that they’re there,” he said. “I mean, that’s a death sentence right there.”