The driver of the trailer truck in which at least 53 migrants died before being abandoned in San Antonio this week did not realize the vehicle’s air conditioning system was broken, federal court documents said.

The detail was contained in records explaining why investigators arrested a man with whom the driver exchanged text messages, in what is believed to be the deadliest migrant smuggling episode on the US-Mexico border.

The alleged driver of the truck, Homero Zamorano Jr, 45, and his alleged correspondent, Christian Martinez, 28, are among four people charged in connection with the discovery of the bodies in an industrial area of south-west San Antonio on Monday night.

Authorities allegedly spotted Zamorano hiding in brush near the truck, pretending to be a passenger. The resident of Pasadena, Texas, was arrested after officers recovered surveillance video of him driving the rig through an immigration checkpoint.

Zamorano’s arrest prompted agents to comb through his texts, finding he had sent messages to Martinez before and after the discovery of the dead migrants.

The texts included a message containing an abbreviation asking “where you at”, sent around the time authorities spotted the rig and the corpses, making authorities suspect Martinez was involved in trying to sneak the migrants across the border illicitly.

A confidential informant told agents of an alleged conversation with Martinez, investigators wrote in court documents filed under oath. During that conversation, Martinez allegedly identified Zamorano as the driver and said he “was unaware the air conditioning unit stopped working and was the reason why the [passengers] died”.

Agents determined that the informant’s cellphone placed him “within several meters” of Martinez during the time of that alleged conversation, investigators wrote.

Both Zamorano and Martinez face charges of plotting to illegally smuggle migrants into the US, leading to their deaths. They could get life in prison or the death penalty.

Two Mexican nationals, Juan Claudio D’Luna Mendez, 23, and Juan Francisco D’Luna Bilbao, 48, were arrested and charged with illegally possessing guns after investigators found them at an address linked to the trailer truck. They face up to 10 years in prison if eventually convicted of those charges.

Authorities were holding all four suspects in custody without bond.

Officials believe the rig at the center of the case was carrying at least 64 migrants from countries such as Mexico, Honduras and Guatemala. At least 40 men and 13 women died and 11 were hospitalized with heat-related conditions. The trailer had traveled through temperatures approaching 100F (38C).