Two men were indicted Wednesday in the case of a hot, airless tractor-trailer rig found last month with 53 dead or dying migrants in San Antonio, officials said.

A federal grand jury in San Antonio indicted Homero Zamorano Jr., 46, and Christian Martinez, 28, both of Pasadena, Texas, on counts of transporting and conspiring to transport migrants illegally resulting in death; and transporting and conspiring to transport migrants illegally resulting in serious injury.

Both remain in federal custody without bond pending trial. Martinez’s attorney, David Shearer of San Antonio, declined to comment on the indictments. A message to Zamorano’s attorney was not immediately returned.

Conviction on the death counts could result in life sentences, but the Attorney General’s Office could authorize prosecutors to seek death penalties. The serious bodily injury counts carry sentences of up to 20 years in prison.

It was the deadliest tragedy to claim the lives of migrants smuggled across the border from Mexico. The truck had been packed with 67 people, and the dead included 27 from Mexico, 14 from Honduras, seven from Guatemala and two from El Salvador, said Francisco Garduño, chief of Mexico’s National Immigration Institute.

The incident happened on a remote San Antonio back road on June 27. Arriving police officers detained Zamorano after spotting him hiding in some nearby brush, according to a statement from the U.S. Attorney’s Office. A search of Zamorano’s cellphone revealed calls with Martinez concerning the smuggling run.

Surveillance video of the 18-wheeler passing through a Border Patrol checkpoint showed the driver matched Zamorano’s description, according to the indictment. One survivor of the journey, a 20-year-old from Guatemala, told The Associated Press that smugglers had covered the trailer’s floor with what she believes was powdered chicken bouillon, apparently to throw off any dogs at the checkpoint.

The tragedy occurred at a time when huge numbers of migrants have been coming to the U.S., many of them taking perilous risks to cross swift rivers and canals and scorching desert landscapes. Migrants were stopped nearly 240,000 times in May, up by one-third from a year ago.

Of the 73 people in the truck, those who died included people from the Mexican states of Guanajuato, Veracruz, Oaxaca, Mexico, Zacatecas, Queretaro, Morelos and Mexico City. Migrants from Honduras and Guatemala also were among those who died in the deadliest known smuggling attempt in the United States.

In 2017, 10 people died after being trapped inside a truck parked at a San Antonio Walmart. In 2003, the bodies of 19 migrants were found in a sweltering truck southeast of the city.