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.