U.S. authorities on Tuesday announced the arrests of four men they say were part of a human smuggling effort last year that ended in the deaths of 53 migrants, including eight children, who were left in a tractor-trailer in the scorching Texas summer.

Authorities said on the anniversary of the June 27, 2022, tragedy that the four Mexican nationals had a planning role in the smuggling operation, and were aware that the trailer's air-conditioning unit was malfunctioning and would not blow cool air to migrants trapped inside during the sweltering three-hour ride from the border city of Laredo to San Antonio.

When the trailer was opened in San Antonio, 48 migrants were already dead. Another 16 were taken to hospitals, where five more died.

It was the deadliest tragedy to claim the lives of migrants smuggled across the border from Mexico. The dead included 27 people from Mexico, 14 from Honduras, seven from Guatemala and two from El Salvador.

The driver and another man were arrested shortly after the migrants were found. They were charged with smuggling resulting in death and conspiracy.

The four new arrests were made on Monday in Houston, San Antonio and Marshall, Texas.

A federal grand jury indictment unsealed Tuesday alleges the men worked with human smuggling operations in Guatemala, Honduras and Mexico, and shared routes, guides, stash houses, trucks and trailers, some of which were stored at a private parking lot in San Antonio.

"Human smugglers prey on migrants' hope for a better life – but their only priority is profit," U.S. Attorney General Merrick Garland said in a statement. "Tragically, 53 people who had been loaded into a tractor-trailer in Texas and endured hours of unimaginable cruelty lost their lives because of this heartless scheme."

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(With input from AP)