Bexar County Judge Nelson Wolff said 46 people were found dead at the scene, and four more died after being taken to the hospital, leaving 12 survivors, including four children. Thirty-nine of the dead were men, and 11 were women.
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SEE RELATED STORY: 50 migrants die after 18-wheeler abandoned in San Antonio heat
San Antonio Mayor Ron Nirenberg said those who died had "families who were likely trying to find a better life."
The city's police chief, Bill McManus, described this case as the deadliest human smuggling incident he could recall in the city.
Southern Texas has long been a busy area for illegal border crossings, and big rigs have become a popular smuggling method in response to heightened border patrol.
"It's important to point out that this is not the first time, nor will it be the last that this happens," said Cesar Espinosa, executive director of Fiel Houston.
On July 23, 2017, 39 undocumented immigrants were found dead in a sweltering trailer at a San Antonio Walmart parking lot. Eight were dead and two others died later at the hospital. The driver was ultimately sentenced to life in prison in 2018.
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Then back in 2003, the bodies of 19 migrants were also found in a scorching truck in Victoria.
RELATED: How America's deadliest smuggling incident unfolded in Victoria, Texas
In response to Monday's incident, Gov. Greg Abbott tweeted in part, "These deaths are on Biden. They are a result of his deadly open border policies. They show the deadly consequences of his refusal to enforce the law."
Espinosa disagrees and believes that if the border was open, migrants would not take such extreme measures to get to the United States. He said if our country's leaders do not create a pathway for people to migrate safely and legally, we will continue to see cases of mass casualties.
WATCH: Advocate says it's not time for fear-mongering after smuggling operation
Advocate says it's not time for fear-mongering after smuggling operation
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"Let's get one thing straight. Out of respect for the families, right now is not the time to start the fear-mongering. It's not the time to start with all the politics," said Espinosa. "Even though we don't condone people coming in that manner, we also take a step back and ask ourselves, 'Why are people coming this way? What makes it so difficult in somebody's home country to put their lives at risk, to put their children's lives at risk, and have them come over this way?'"
The Mexican foreign secretary tweeted Tuesday morning that at least 22 migrants have been identified as Mexican, seven as Guatemalan, and two as Honduran.
Espinosa said that based on experience, his organization anticipates the possibility of receiving calls from individuals expecting a relative who may have died on that tractor trailer.
Homeland Security Investigations reported they have three people in custody who are believed to be part of the smuggling conspiracy.
President Joe Biden released a statement Tuesday morning that reads in part, "This incident underscores the need to go after the multi-billion dollar criminal smuggling industry preying on migrants and leading to far too many innocent deaths. In Los Angeles two weeks ago, I announced that the United States has launched a first-of-its-kind anti-smuggling campaign with our regional partners. In the first three months, we have made over 2,400 arrests, and that work will only intensify in the months ahead."
Fiel Houston will be holding a vigil for the 50 people who died on Tuesday at 7 p.m. at 6610 Harwin Drive.