The death toll of the migrant filled tractor-trailer abandoned in the San Antonio heat continued to grow this week. With 53 migrants dead, this incident had become the deadliest human smuggling incident in U.S. history, according to Craig Larrabee, acting special agent in charge of Homeland Security Investigations in San Antonio.

Of the 53 people that died, 40 were identified as male and 13 as female by the Bexar County Public Information Office; on Wednesday, the office said they had potentially identified 37 of the casualties and were working on confirming the remaining identities.

Authorities stated that almost all migrants who lost their lives most likely suffered from heat exhaustion and heat stroke. According to Immigration and Customs Enforcement, victims who have been identified so far came from Mexico, Guatemala, and Honduras.

The driver of the tractor-trailer and three other human smugglers were arrested according to the Department of Justice; all four remain in federal custody.

USAToday reports on the smugglers:

The Justice Department identified the driver as Homero Zamorano Jr., 45, of Pasadena, Texas. He was arrested Wednesday and has been charged with one count of smuggling resulting in death.

Christian Martinez, 28, was also charged in connection with the smuggling. Investigators executed a search warrant on Zamorano's phone and said they found that he had communicated with Martinez about the smuggling conspiracy beforehand. Martinez faces a charge of conspiracy to smuggle, resulting in death.

Juan Claudio D'Luna-Mendez, 23, and Juan Francisco D'Luna-Bilbao, 48, are both facing a charge of possessing a weapon while in the U.S. illegally, federal charging documents filed in the Western District of Texas show. The two men, who are Mexican citizens, haven't been charged in the suspected smuggling conspiracy, court records show. [But have admitted to overstaying their visas.]

Authorities say that DHS is leading the investigation into the suspected smuggling operation.

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