The death toll of migrants who died after they were abandoned in the back of a tractor-trailer that was discovered Monday in San Antonio rose to 53 on Wednesday, according to the Bexar County medical examiner. More than a dozen others were taken to local hospitals with heat-related injuries.
Among the dead were 40 men and 13 women, the medical examiner said.
Federal law enforcement officials confirmed to CBS News Homeland Security and Justice reporter Nicole Sganga this appears to be the deadliest human smuggling case in modern U.S. history.
Not all of the victims have been identified, and some of the victims could be under 18 years old, county spokesman Tom Peine told reporters. During a Tuesday afternoon press conference, officials with Bexar County, Texas, which includes San Antonio, said the death toll had risen to 51.
"This work will take days, if not longer," Peine said.
On Tuesday morning, Mexican Foreign Relations Secretary Marcelo Ebrard said on Twitter, citing U.S. authorities, that the victims included 22 Mexicans, 7 Guatemalans and two Hondurans. Investigators are still determining the nationalities of those who died or were hospitalized, Sganga reports.
Three individuals believed to be part of the suspected smuggling conspiracy are being detained by the federal agency Homeland Security Investigations, a spokesperson with U.S. Immigration and Customs Enforcement, or ICE, told CBS News immigration reporter Camilo Montoya-Galvez.
Officials said Monday 12 adults and four children were taken to local hospitals for treatment. Baptist hospital confirmed to CBS News on Tuesday that two of its patients have died.
During Tuesday's press conference, County Commissioner Rebeca Clay-Flores said the county's hospital system was treating two patients, a 23-year-old woman from Guatemala and a teenage boy.
"We are asking for prayers for him," Clay-Flores said. She said the woman's condition was improving.
Police Chief William McManus said at a press conference Monday that authorities received a call at approximately 5:50 p.m. local time from a worker who had heard cries for help and found the trailer with its doors partially open. The worker found a number of dead bodies inside the trailer, McManus said.
"The scene was tragic beyond words," McManus. "I don't understand how anyone could be so callous as to allow it happen and run from the scene."
San Antonio Fire Chief Charles Hood said the 16 people taken to the hospital were all conscious when they were found, though they were "weak" and would have been unable to leave the trailer on their own. He said all were "hot to the touch" and suffering from heat-related injuries like heat exhaustion and heat stroke.
He said there were no signs of water or a working air conditioner in the truck, and authorities said it's not clear how long the group was in the truck before they were found.
The victims were a mix of men and women and varied in age, officials said.
Hood said first responders were being debriefed to help them process what they witnessed.
"You're not supposed to open up a truck and see stacks of bodies in there," he said. "None of us come to work imagining that."
San Antonio Mayor Ron Nirenberg called the situation "a horrific human tragedy."
Homeland Security Investigations is leading the investigation into the incident and will work in conjunction with the San Antonio Police Department, law enforcement officials told CBS News.
DHS said in a statement it is "horrified at this tragic loss of life near San Antonio. This speaks to the desperation of migrants who would put their lives in the hands of callous human smugglers who show no regard for human life."
An ICE spokesperson said in a statement that Homeland Security Investigations responded to the scene after receiving a call from the police department. The spokesperson said more details will be released as the investigation continues.
More than 20 emergency vehicles responded to the scene, CBS affiliate KENS-TV reported.
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