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.
– 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.