Privacy advocates worried about x-ray scanners making their way around U.S. airports may be surprised to know the technology is also making its way onto America's streets.

The Department of Homeland Security, the U.S. military and even local law enforcement agencies are buying and deploying mobile X-ray vans that can see into the interior of vehicles around them.

The Z Backscatter Van (ZBV), manufactured by American Science and Engineering (AS&E), can be used to detect contraband such as car bombs, drugs and people in hiding.

But the vans, which can also see through clothing and into some buildings, are raising privacy concerns as well as questions about health risks -- and what might happen if the technology gets into the wrong hands.

FoxNews.com was given a rare ride-along in a ZBV at a U.S. seaport in Elizabeth, N.J.

Click here to see FoxNews.com's video report on the ZBV.

Like airport scanners, the ZBVs use Z Backscatter technology to detect materials that contain low atomic numbers. This allows them to detect organic matter that doesn't show up well in traditional X-ray images -- including explosives and plastic weapons – in addition to metal and other materials.

The technology also works in such a way that the X-Ray mechanism has no need for a detector on the far side of an object, allowing it to be extremely mobile, versatile and capable of being into a commercially available van.

Once equipped, the van -- which looks like a standard delivery van -- takes less than 15 seconds to scan a vehicle; it can be operated remotely from more than 1,500 feet and can be equipped with optional technology to identify radioactivity as well.

The Z Backscatter vans range in cost from $729,000 to $825,000. The DHS says they have been a huge asset at the nation's ports and borders, and at major crowd events like the Super bowl.

"Using the ZBV vans over the past couple of years, we've gotten over a thousand seizures and 89,000 pounds worth of narcotics, approximately $4 million worth of currency, and we've also uncovered 10 or 11 undeclared aliens within vehicles," said Patrick Simmons, Director of Non-Intrusive Inspection at Customs and Border Protection. "Again, we don't purposely scan for people, but if they're in there hiding, the ZBV will be able to spot them."

But according to the AS&E website, ZBVs also can peer through clothing and into "lightly constructed" buildings, raising serious concerns among privacy advocates.

"A van that can drive down the street and look through people's clothes, look into vehicles and even peer into your home? I think that's an invasion of privacy and not what we should be doing," Utah Congressman Jason Chaffetz told FoxNews.com.

AS&E says the system's primary purpose is to screen vehicles and containers for contraband and security threats, and it doesn't violate a person's privacy in the rare event an individual is scanned.

"If a person, such as an illegal stowaway, is present in the vehicle or container being scanned, the system creates only a silhouette with no facial or body detail," the website says. "The system cannot be used to identify an individual, or the race or age of the person."

But Chaffetz, who is working on legislation aimed at limiting the use of the backscatter body scanners in airports, says the vans need restrictions.

"There's an appropriate use for these machines -- at ports for instance, coming across the border and inspecting vehicles, hostage situations. But the company that develops these vans says they've sold more than 500 of these roving vans and I don't know who's purchased them," he said. "I think we need to know."

But it's hard to know exactly who owns ZBVs, because AS&E has never fully disclosed its buyers.

"Due to the highly sensitive nature of the markets that our products serve, AS&E respects the individual requests of our customers to be confidential," the company says on its website.

In a June 2009 press release the company said it sold 400 ZBVs to 85 customers in 46 countries. The company has since raised that number to 500, saying some of those purchases are now going to local U.S. law enforcement agencies.

A search of the site and additional company press releases showed that its clients include:

- U.S. Customs and Border Protection

- U.S. Transportation Security Administration

- U.S. Domestic Nuclear Detection Office

- U.S. Department of Defense, including U.S. Navy, U.S. Air Force, and U.S. Marines

- North Atlantic Treaty Organization

- Royal Thai Police

- HM Customs & Excise (U.K.)

- New Zealand Customs Service

- Hong Kong Customs

- Singapore's Immigration and Checkpoints Authority

- National Customs Agency of Bulgaria

Other releases are more vague, however, identifying the purchasers only as "the U.S government," a "Latin American customs agency," an "international government agency," "U.S. law enforcement officials," a "South American government," a "Middle Eastern country," a "Middle Eastern government," a "Middle East government agency," a "Middle East law enforcement agency," a "South American law enforcement agency," a "new African customer," a "European Union (EU) and an Asia Pacific (APAC) client," and a "Middle Eastern customer."

That ambiguity has Chaffetz worried.

"In a hostage situation you want to be able to peer into the house, I buy that," Chaffetz said. "But in the hands of a private individual? That scares the living daylights out of me.

"It was cute when Superman had these powers, but now that it's reality we need to think through how we're going to do this. I don't want a stranger peering through the walls of my home watching my kids."

Jay Stanley, a senior policy analyst for the ACLU, says knowing the vans are being used on the streets, even by local law enforcement agencies, is troublesome.

"We don't know who all those agencies are or what they're using them for," he said. "…This technology has the potential to be a tremendous invasion of people's privacy."

FoxNews.com asked police departments in the Department of Homeland Security's five highest-ranked terror risk areas -- New York, Los Angeles, Washington, D.C., San Francisco and Chicago -- if they use ZBVs. The New York Police Department confirmed it does but wouldn't say how.

"Yes, we do utilize this technology; however, we're unable to divulge any specifics of the use due to confidentiality concerns," Det. Cheryl Crispin of the NYPD told FoxNews.com.

San Francisco PD said they do not use the vans. The other departments did not respond.

Constitution Attorney Noel Francisco says most, if not all, state privacy laws would prohibit individuals or private companies from abusing the vans, while the Fourth Amendment prohibits law enforcement agencies from doing the same.

"If you take this thing and point it at somebody's house or point it at somebody's car, you're engaging in a search of that individual," Francisco said. "You can't do that without a warrant or probable cause."

But since it's virtually impossible to detect a ZBV search, Francisco said the law would be harder to enforce, raising the need for more guidelines.

"It's certainly very useful for certain types of things, like anti-terror detection, but we may want to put into place some kind of guidelines on where and when they can use that. Much like wiretapping," he said.

Another issue with the machines is their potential health hazards.

"So long as a person is somewhere away, like tens of feet, the dose isn't that high, it's very, very low indeed," Arizona State University Professor Peter Rez, an expert in radiation physics, told FoxNews.com. But if a person were to walk next to the van while it was scanning, Rez said, "Then I would start getting worried."

AS&E says the system is safe for operators and subjects, and that "one scan of the ZBV is equivalent to flying in an airplane at altitude for two minutes."

Rez says the levels would be fine in most cases, but in certain circumstances they could pose a small risk.

"Let's assume a pregnant woman pushing a stroller slowly walks by this van and is quite close to the side of the van, maybe within one to two feet. This woman and her baby could receive a few micro Sv, still not a high number but more than the NS 43.17 standard allows," he told FoxNews.com.

Kevin McCabe, Chief of U.S. Customs and Border Protection's Antiterrorism Contraband Enforcement Team, says DHS takes all the proper precautions when it comes to safety.

"When we utilize these machines we actually have people deployed around our perimeter to ensure that no one is radiated inadvertently… and my general understanding is even if you were exposed to a dosage from one of these machines, it would be equivalent to a chest x-ray or less," McCabe told FoxNews.com.

Simmons adds that given the limited image the ZBVs show of a person, the only reason to scan someone would be to detect contraband, and given the limited image it would show of a home, there's little use for them outside their intended security purposes.

"You'd have to be inches away from the house, you'd be better off just looking into the window," Simmons said. "And, again, a house has a pretty thick foundation. If it was brick or something like that I'm not sure the ZBV's even powerful enough to get through that."

Rez says one of his students reported using a ZBV at the U.N. while he was serving in the military.

"It was a secondary screening mechanism for trucks going into a loading dock, but it was on a public street and they were just scanning people and nobody was being told this was going on," Rez told FoxNews.com. "That kind of shocked me. …I think they're being used in a more widespread manner than people would have one believe."

Regardless of whether regulations are passed in relation to the vans, McCabe says the benefits far outweigh the risks.

"If local law enforcement had intelligence information that something was going to happen, there was going to be some sort of an attack and they have identified an area… it would be very useful to know where a problem might be before it happens," he said.