By the time they climbed up into the pitch-black truck, they had already crossed the Rio Grande on rafts and walked all night through wild brush land.
The air was hot inside the 18-wheeler, and there was no food or water.
But just before they set off, according to an account of the fatal journey filed Monday in federal court, a man opened the door. He reassured the scores of migrants huddled inside that the vehicle had refrigeration.
Don't worry about the journey, he said.
A few hours later, the truck driver told a federal agent, he opened the door to find "bodies just lying on the floor like meat."
James Matthew Bradley Jr., 60, of Clearwater, Fla., could face the death penalty for his role in the deaths of 10 migrants who perished after they were crammed into the hot tractor-trailer.
In a federal complaint filed Monday, Bradley was charged under federal law with one count of "transporting illegal aliens," a crime punishable by life imprisonment or death if the crime results in a death.
Thirty-nine people were discovered in and around the truck early Sunday after a disoriented man approached an employee for water in a Wal-Mart parking lot just off Interstate 35, about 150 miles from the U.S.-Mexico border.
Eight men were pronounced dead at the scene Sunday morning and are believed to have suffered from heat exposure and asphyxiation.
Among the dead was a so-called Dreamer, a migrant who had been brought to the United States as a young child. Frank Guisseppe Fuentes, 20, spent much of his life in the U.S. and had crossed the border in an attempt to reunite with family members living in Maryland after he was deported to Guatemala City, Jose Barillas, the Guatemalan consul general in Houston, told Univision.
Barillas said Fuentes had been a beneficiary of Deferred Action for Childhood Arrivals, a program created by former President Obama that grants work permits and temporary protection from deportation to those whose parents brought them to the U.S. as children. Fuentes had lost his protected status after committing crimes, Barillas said. He did not say what those crimes were.
"He was trying to get back," Barillas said.
Seventeen of the vehicle's occupants were rushed to hospitals with serious or critical injuries. An additional 13 had non-life-threatening injuries. Two men have since died at hospitals, according to the U.S. attorney's office.
"These human smugglers crammed more than 100 people into a tractor trailer in the stifling Texas summer heat," Thomas Homan, acting director of Immigration and Customs Enforcement, said in a statement. "Human smugglers have repeatedly demonstrated that they have absolutely no regard for human life."
During the journey, one of the men trapped in the sweltering truck made a desperate call home.
"There's no oxygen," Mario Alberto Ramirez Mendes, 23, told his family members in Calvillo, a small city in central Mexico known as the country's top producer of guavas.
"We can't breathe," he said.
Ramirez left home two weeks ago with his 18-year-old nephew, Jhonny Serna Ramirez, and two others who hoped to cross into the U.S.
The group waited for about a week at a hotel on the border before smugglers helped them cross the Rio Grande, according to Mario Ramirez, Jhonny's 17-year-old brother.
In interviews with Homeland Security agents Sunday, some of the truck's passengers said they had paid "protection" money to the Mexican criminal cartel Zetas to cross the border. They estimated that as many as 70 to 200 people may initially have been crammed on board.
A San Antonio police officer found multiple people around the rear of the trailer, according to an affidavit filed by Homeland Security Investigations Special Agent James Lara.
When the officer shone a light into the cab of the trailer, Bradley emerged from the cab's rear camper, the agent reported. He told the officer that he was driving the trailer from Schaller, Iowa, to Brownsville, Texas, and that he was unaware of the trailer's cargo.
Bradley said he heard movement in his truck only when he exited the vehicle for a bathroom break, according to the affidavit. After opening the door at the back of the trailer, he said, he attempted to administer aid.
During further questioning by Homeland Security Investigations agents, Bradley said he was traveling from Laredo, Texas, to deliver the truck to someone who had purchased it. As he exited the vehicle, the affidavit said, he heard "banging and shaking" in the trailer. When he opened the door, he said, he was surprised when "he was run over by 'Spanish' people and knocked to the ground."
About 30 to 40 people ran from the trailer, Lara wrote in the affidavit. Immediately, Bradley told him, he realized at least one person was dead. He said that he knew the trailer refrigeration system did not work and that the four main vent holes were likely clogged.
He called his wife but did not call 911, he told agents.
Several of the immigrants taken from the trailer told agents they had been smuggled across the river near Laredo in different groups and then held at various stash houses.
One immigrant, a Mexican citizen from Aguascalientes state, told agents he had traveled via Nuevo Laredo, Mexico, and had arranged to pay smugglers $5,500 when he arrived in San Antonio.
After waiting with 28 people until 8 p.m. Friday to be smuggled across the river, smugglers told him that people associated with the Zetas would charge about $700 for protection and to cross by raft. The group crossed the river in three trips and then walked throughout the night.
About 9 a.m., his group was picked up by a silver Chevrolet Silverado truck and taken to the trailer. About 70 people were already inside, he estimated.
Everything was OK for the first hour. But then some passengers started having difficulty breathing and began to pass out, agents learned from their interviews.
People began hitting the trailer walls in an effort to get the driver's attention.
One by one, people started taking turns breathing from a ventilation hole in a wall.
At the end of their journey, when they finally pulled in to the Wal-Mart parking lot, the driver slammed the brakes abruptly, causing many passengers to fall over. The rear doors were opened and, as the driver described it, people started to "swarm out."
Six black SUVs were waiting to pick up people. They left as soon as they filled with passengers.
The Mexican Foreign Ministry said that at least four of the dead were Mexican citizens and that 21 of those hospitalized are Mexican.
Two Guatemalans survived and are in stable condition.
After receiving a panicked call from Ramirez late Saturday night, family members began frantically dialing the phone number of one of the smugglers in hopes that he could communicate with the driver of the truck, Mario Ramirez said.
Their efforts were fruitless. The next day, the family found out that Ramirez was OK. But Serna, his nephew, is gravely ill.
Serna, who is being treated at a hospital in San Antonio, is still unconscious, his brother said, and has suffered serious kidney damage caused by severe dehydration. His parents are traveling to the U.S. on Monday on humanitarian visas to be with him.
"He is strong," his brother said in an interview Monday. "But we feel desperate."
Serna left for the U.S. because he was unable to find well-paying work after leaving school, his brother said.
"He only went to fulfill his American dream," Ramirez said.
Adan Valdivia Lopez, mayor of Calvillo, said that at least four residents of his city were on the tractor-trailer.
"There is not a single family here that does not have a relative in the United States," Valdivia Lopez said by phone.
The parents of those who survived the incident, he said, are worried about what U.S. immigration authorities may do to them.
"The parents were very concerned primarily about the health of their children, but also about the treatment they will be given if they are deported," he said.
Smuggling migrants in tractor-trailers is a relatively common practice along the Southwest border.
In May 2003, 19 people died of dehydration, hyperthermia, suffocation and mechanical asphyxia after they were abandoned in a trailer truck at a truck stop in Victoria, Texas. The driver in that case, Tyrone Mapletoft Williams, was convicted and is serving a sentence of nearly 34 years in prison.
In Williams' case, the U.S. 5th Circuit Court of Appeals ruled in 2010 that simply leaving migrants inside the truck was not punishable by death. "Williams' conduct during the smuggling trip, despicable as it was, fell short of the statutory minimum to subject Williams to the possibility of a death sentence," the court ruled.
Jarvie reported from Atlanta and Linthicum reported from Mexico City. Times staff writer Matt Pearce in Los Angeles and special correspondent Cecilia Sanchez in Mexico City contributed to this story.
7:30 p.m. The article was updated with a statement from the Mexican foreign ministry regarding the number of Mexican citizens who were in the truck.
5:46 p.m.: The article was updated with additional details from a federal law enforcement affidavit and interviews with relatives of the migrants.
12:05 a.m.: The article was updated throughout with Times staff reporting.