SAN ANTONIO (Nexstar) — The death toll of migrants who died after being abandoned inside a truck trailer in the sweltering Texas heat has risen to 53, Bexar County officials reported Wednesday.

On Monday, officials found an abandoned 18-wheeler in an area on Quintana Road in southwest San Antonio. Dozens of migrants were found dead inside the truck without air conditioning.

It is the deadliest tragedy to claim the lives of migrants smuggled across the border from Mexico in recent history.

“On June 27, Homeland Security Investigations (HSI) responded to a call from San Antonio Police Department (SAPD) in reference to an alleged human smuggling event involving a tractor trailer,” a spokesperson for U.S. Immigration and Customs Enforcement said in an email statement. “Upon arrival at the scene, HSI confirmed more than 40 deceased individuals. HSI San Antonio has initiated an investigation with support of SAPD.”

After receiving 911 calls from “concerned citizens,” San Antonio police officers arrived on the scene Monday and found multiple migrants still inside the tractor trailer, some on the ground, some in a nearby brush. Many of them were already deceased and some were incapacitated, according to a news release from the U.S. Department of Justice (DoJ).

Federal investigators have arrested four men in relation to the incident.

Homero Zamorano, 45, was arrested on Wednesday on criminal charges related to his alleged involvement in “alien smuggling resulting in death.” SAPD officers were led to a location where they found Zamorano hiding after he fled the scene, according to the press release. Federal officials arrested him Wednesday on charges related to “alien smuggling resulting in death.”

According to an affidavit, investigators traced the truck’s registration plate to a residence in San Antonio, where officers surveilled the residence. There, they found two men from Mexico — Juan Francisco D’Luna-Bilbao, 48, and Juan Claudio D’Luna-Mendez, 23 — leaving the home in a truck.

Authorities arrested the men on possession of weapons charges, after D’Luna-Mendez confessed to having a handgun, according to the criminal complaints. Officers obtained a search warrant and searched the home linked to the truck’s registration, where they found more guns.

Christian Martinez, 28, is charged with conspiracy to transport illegal aliens resulting in death. According to court documents, communications between Zamorano and Martinez in which they discussed human smuggling were found.

If convicted, Martinez faces up to life in prison or the death penalty, according to the DoJ.

Mexico’s Secretary of Foreign Affairs Marcelo Ebrard said Tuesday at least 22 of those who died were from Mexico. Another seven were from Guatemala, and at least two were from Honduras. Authorities are working to identify the others. He said Mexico is investigating alongside the U.S. Department of Homeland Security.

At a press conference in Eagle Pass Wednesday, Gov. Greg Abbott said Texas will immediately start additional truck checkpoints to target trucks like the one found in San Antonio.

Abbott said the Texas Department of Public Safety is also creating two strike teams, each consisting of 20 troopers who will be “deployed immediately” to high-traffic crossing areas. DPS Director Steven McCraw said the teams will be along “smuggling corridors” to conduct surveillance and look for stolen vehicles.

The death count was the highest ever from a smuggling attempt in the United States, according to Craig Larrabee, acting special agent in charge of Homeland Security Investigations in San Antonio.

“This is a horror that surpasses anything we’ve experienced before,” San Antonio Mayor Ron Nirenberg said. “And it’s sadly a preventable tragedy.”