The suspected driver of a truck where 53 migrants died from heat in Texas did not know that the air conditioner had stopped working, court documents say.
Homero Zamorano, 45, was found hiding near the truck on Monday, officials say. He is one of four people charged.
Mr Zamorano and alleged conspirator Christian Martínez, 28, are accused of sending text messages to each other before and after the truck was found.
It is the deadliest human trafficking incident in US history.
Several children found in the vehicle are still being treated in hospital. The victims were from Mexico, Honduras, El Salvador and Guatemala.
Mr Zamorano was found hiding in bushes. Mexican officials say he initially tried to pass himself off as one of the survivors.
He was arrested when surveillance photos showed him driving the truck past a US Border Patrol checkpoint in Laredo, Texas.
According to federal court documents, a confidential government informant working for Immigration and Customs Enforcement and Texas police told officials that Mr Zamorano and Mr Martínez had spoken after the deaths were reported.
Mr Martínez allegedly said: "The driver was unaware the air conditioning unit stopped working and was the reason why the individuals died."
The documents add that the informant and Mr Martínez were standing within several metres of each other when the conversation took place.
Both suspects face the death penalty if found guilty of smuggling and conspiracy charges.
Two other men accused of being involved, Juan Claudio D'Luna-Méndez and Francisco D'Luna-Bilbao, have been charged with firearms possession and with being in the US illegally. Both are Mexican citizens.
The victims include 27 Mexican citizens, as well as 14 Hondurans, seven Guatemalans and two Salvadorans.
However, Mexican authorities say a total of 67 migrants were inside the lorry, while prosecutors in San Antonio have put the number at 64.
On Friday, the Bexar County medical examiner's office said it had "conclusively identified" six of the 53 victims from Monday's incident.
In addition, 42 "potential identifications" have been made. Five remain unidentified.
Also on Friday, the Bexar County Sheriff's office began investigating an apparently abandoned 18-wheel truck found in a residential area just a few miles north of where the first vehicle was found.
About 13 migrants were found in the most recent truck, officials tell CBS News.
"Preliminarily, it appears no individuals have suffered any major injuries," the sheriff's office posted on Facebook, adding that more details would be released once they were available.
People-smuggling is a major industry along the US-Mexico border.
In May, a record 239,000 undocumented migrants were detained crossing into the country from Mexico.
Mass deaths of this kind have been uncovered on a number of occasions around the world in recent times. Among the worst cases are:
- Austria, 27 August 2015: An abandoned Hungarian-registered lorry was found containing the bodies of 71 Iraqi, Syrian and Afghan migrants. Three years later, four men received 25-year jail terms in Hungary in connection with the deaths
- Libya, 20 February 2017: Dozens of African migrants were found locked in a shipping container, including 13 people who had suffocated to death
- UK, 23 October 2019: A total of 39 Vietnamese migrants were found dead in a lorry trailer in Essex. Four men were jailed for their manslaughter in January 2021