Police in Mexico's southern Chiapas state found 513 migrants on Tuesday inside two trailer trucks bound for the United States, and said they had been transported in dangerously crowded conditions. 
 
 Migrants who were found in two trailer trucks bound for the United States, sit under the guard of a policeman in Tuxtla Gutierrez, Mexico, Tuesday May 17, 2011. Chiapas authorities say they rescued 513... (Associated Press) 
 
 Some of the immigrants were suffering from dehydration after traveling for hours clinging to cargo ropes strung inside the containers to keep them upright as the trucks bounced along from the Guatemalan border, and allow more migrants to be more crammed in on the floor. 
 
 The trucks had air holes punched in the tops of the containers, but migrants interviewed at the state prosecutors' office said they lacked air and water. The trucks were bound for the central city of Puebla, where the migrants said they had been told they would be loaded aboard a second set of vehicles for the trip to the U.S. border. 
 
 "We were suffering, it was very hot and we were clinging to the ropes," said Mario, a 23-year-old Honduran migrant who identified himself only by his first name, for security reasons. Mexico's National Human Rights Commission says thousands of undocumented migrants are kidnapped and held for ransom by drug gangs in Mexico each year. 
 
 None of the migrants would say whether any drug gang had been involved in the mass smuggling scheme broken up early Tuesday when Chiapas state police discovered the migrants while using X-ray equipment on the trucks at a checkpoint in the outskirts of city of Tuxtla Gutierrez. 
 
 The migrants said the smugglers were charging them about $7,000 apiece to get them into the United States. A Guatemalan migrant who identified himself as Juan said remaining in his hometown in Guatemala was not an option, noting "a lot of us are Indians, and we can't stay in our homes. There is no work, and there's nothing to eat." 
 
 An agent for the National Immigration Institute who was not authorized to be quoted by name said it was the largest shipment of migrants detained in Mexico in recent years. 
 
 Police also arrested four people accused of smuggling the migrants, who are from Central and South America and Asia, Chiapas state prosecutors said in a statement. 
 
 The alleged smugglers tried to escape police but were chased down and captured, prosecutors said. 
 
 The immigration institute said in a statement that 410 of the migrants were from Guatemala, 47 from El Salvador, 32 from Ecuador, 12 from India, six from Nepal, three from China and one each from Japan, the Dominican Republic and Honduras. There were 32 women and four children among them. 
 
 In January, Chiapas state authorities discovered 219 migrants squeezed into a trailer truck. 
 
 Most of those migrants were from Central America but six were from Sri Lanka and four from Nepal. ||||| Government of Chiapas, via European Pressphoto Agency 
 
 When police ran X-ray scanners over two cargo trucks at a checkpoint in southeastern Mexico on Tuesday, they made a surprising discovery: Inside the trailer were the ghostly shadows of 513 migrants — some suffering from dehydration — packed together in near-suffocating conditions. 
 
 The police released an image of the harrowing scan, which shows how migrants sat in tight bundles or stood clutching cargo straps for hours of clandestine travel from beyond Mexico’s border with Guatemala. 
 
 Their capture thwarted what appeared to be one of the largest single shipments of illegal migrants found in recent years, The Associated Press reported. And it drew attention to the increasing use of X-ray scanners by border security and police officers to inspect vehicles on the road — not to mention the power of these machines to create frightful, captivating images. 
 
 Scanners of the type used to inspect the trucks in Mexico, known as backscatter scanners, are widely employed to check cargo for weapons and drugs, and also to catch human smuggling. They are similar to the ones now frequently used at airports around the United States to screen passengers. Critics have objected that the images created by these machines — which penetrate normal clothing and can be highly detailed — are a violation of travelers’ privacy. (Remember the failed protest against the scanners last year?) 
 
 No similar controversy attends their use on cargo, though. Fixed and mobile scanners have been used for years everywhere from Europe to the war in Iraq for that purpose. 
 
 Here’s a video of one type of scanner inspecting trucks in Chile (the audio has been removed): 
 
 As Andy Greenberg wrote on Forbes.com last year, “the same technology, capable of seeing through clothes and walls, has also been rolling out on U.S. streets.” He continues: 
 
 American Science & Engineering, a company based in Billerica, Massachusetts, has sold U.S. and foreign government agencies more than 500 backscatter X-ray scanners mounted in vans that can be driven past neighboring vehicles to see their contents, Joe Reiss, a vice president of marketing at the company told me in an interview. While the biggest buyer of AS&E’s machines over the last seven years has been the Department of Defense operations in Afghanistan and Iraq, Reiss says law enforcement agencies have also deployed the vans to search for vehicle-based bombs in the U.S. 
 
 Still, the image captured on Tuesday is among the starkest to surface publicly so far. It opens an eerie black-and-white window into a world where $7,000 buys a small patch of space in a cramped truck with breathing holes punched in the roof, but no guarantee of making it all the way to the United States. 
 
 This post has been revised to reflect the following correction: 
 
 Correction: May 18, 2011 
 
 An earlier version of this post's headline mistakenly referred to the migrants as Mexican. ||||| San Antonio police officers investigate the scene Sunday, July 23, 2017, where eight people were found dead in a tractor-trailer loaded with at least 30 others outside a Walmart store in stifling summer... (Associated Press) 
 
 SAN ANTONIO (AP) — The driver of a broiling tractor-trailer found packed with immigrants outside a Walmart in San Antonio was charged Monday in the deaths of 10 of his passengers and could face the death penalty. 
 
 James Matthew Bradley Jr., 60, of Clearwater, Florida, awaited an appearance in federal court later in the day to face charges of illegally transporting immigrants for financial gain, resulting in death. 
 
 Authorities discovered eight bodies inside the crowded 18-wheeler parked in the summer heat, and two more victims died at the hospital. Officials feared the death toll could rise because nearly 20 others rescued from the truck were in dire condition, many suffering from extreme dehydration and heatstroke. 
 
 "We're looking at a human-trafficking crime," San Antonio Police Chief William McManus said Sunday, calling it "a horrific tragedy." 
 
 It was not immediately known whether Bradley had an attorney who could speak on his behalf. 
 
 Authorities would not say whether the trailer was locked when they arrived, but they said it had no working air conditioning. 
 
 The victims "were very hot to the touch. So these people were in this trailer without any signs of any type of water," Fire Chief Charles Hood said. 
 
 It was the latest smuggling-by-truck operation to end in tragedy. In one of the worst cases on record in the U.S., 19 immigrants locked inside a stifling rig died in Victoria, Texas, in 2003. 
 
 Based on initial interviews with survivors of the San Antonio case, more than 100 people may have been packed into the back of the truck at one point in its journey, said Thomas Homan, acting director of U.S. Immigration and Customs Enforcement. Officials said 39 people were inside when rescuers arrived, and the rest were believed to have escaped or hitched rides to their next destination. 
 
 At least some of those in the truck were from Mexico and Guatemala, according to diplomats from the two countries. Four of the survivors appeared to be between 10 and 17 years old, Homan said. 
 
 Investigators gave no details on where the rig began its journey or where it was headed. But Homan said it was unlikely the truck was used to carry the immigrants across the border into the United States. He said people from Latin America who rely on smuggling networks typically cross the border on foot and are then picked up by a driver. 
 
 "Even though they have the driver in custody, I can guarantee you there's going to be many more people we're looking for to prosecute," Homan said. 
 
 The truck had an Iowa license plate and was registered to Pyle Transportation Inc. of Schaller, Iowa. A company official did not immediately respond to a phone message seeking comment. 
 
 San Antonio is about a 150-mile (240-kilometer) drive from the Mexican border. The temperature in San Antonio reached 101 degrees (38 Celsius) on Saturday and didn't dip below 90 degrees (32 C) until after 10 p.m. 
 
 The tragedy came to light after a person from the truck approached a Walmart employee and asked for water late Saturday night or early Sunday morning, said McManus, the police chief. The employee gave the person water and then called police. 
 
 On Sunday evening, about 100 people gathered at a San Antonio church for a vigil to mourn the dead. 
 
 Immigrants' rights activists and church officials held up handmade signs reading "Who here is not an immigrant" and "No human is illegal." 
 
 Those gathered held a moment of silence, then gave speeches blaming federal and Texas authorities' hard-line immigration policies for contributing to the deaths by forcing immigrants to take greater risks to reach the U.S. 
 
 "These tragedies are compounded when it's incredibly dangerous and incredibly expensive and we push migration into the hands of illicit actors," immigration activist Bob Libal said in a telephone interview. 
 
 In the May 2003 case, the immigrants were being taken from South Texas to Houston. Prosecutors said the driver heard them begging and screaming for their lives but refused to free them. The driver was sentenced to nearly 34 years in prison. 
 
 The Border Patrol has reported at least four truck seizures this month in and around Laredo, Texas. On July 7, agents found 72 people crammed into a truck with no means of escape, the agency said. They were from Mexico, Ecuador, Guatemala and El Salvador. 
 
 Authorities in Mexico have also made a number of similar discoveries over the years. 
 
 Last December, they found 110 migrants trapped and suffocating inside a truck in the state of Veracruz. Last October, also in Veracruz state, four migrants suffocated in a truck carrying 55 people. 
 
 ___ 
 
 Associated Press writers Mike Graczyk in Houston, Elliot Spagat in San Diego, Peter Orsi in Mexico City and Frank Bajak in San Antonio contributed to this report. 
 
 ___ 
 
 This story has been corrected to show that people at the vigil in San Antonio held signs saying "No human is illegal," not "No human is legal." 
 
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– Mexican police saw something shocking when they aimed their X-ray scanners at a pair of tractor trailers at a checkpoint in Chiapas. The trucks were jammed full of 513 illegal migrants, many of them suffering from dehydration, the AP reports. Though air holes had been cut in the top of the trucks, the migrants said they felt suffocated as well. “We were suffering,” one said. “It was very hot, and we were clinging to the ropes.” The migrants hailed from throughout South America, Central America, and even Asia, and were bound for the US. Some said they were charged around $7,000 for the trip. Mexican authorities say it was their largest migrant smuggling bust in years. The type of scanner that spotted the migrants was, interestingly enough, similar to the devices so reviled by airline passengers, the New York Times notes.