Just because a used car is cheap and seems OK during a test drive doesn't mean it's safe to buy.

Criminals have devised ways to artfully conceal structural damage on used cars, allowing vehicles to be sold for a profit even though they may have been shoddily rebuilt after an accident or submerged in ocean water during a hurricane.

When insurance companies write off a vehicle as a "total loss" after an accident or other event like a flood or hail storm, the law in most states requires the vehicle's title of ownership to be given a "brand." That brand permanently marks the car as damaged goods to all potential future owners, but there are ways for it to be washed away. In the old days, it was done with chemicals. Now, photo editing software and digital scanners are used to print new titles. Or, cars can simply be re-registered in different states until the brand falls away.

Because there is no national titling law, incongruous state laws create opportunities for scammers to "title wash" wrecked cars, making them appear undamaged. Not only are consumers being ripped off, but they're also putting themselves and others in danger when they unknowingly drive a salvaged car on U.S. highways.

The scam appears to happen on a large scale: According to the vehicle history provider CarFax, 800,000 cars in the U.S. -- including at least 500 taxis -- have been "title washed" to conceal their troubled histories.

Joanna Zumhagen, a young professional living near Washington, D.C., knows the risks of buying a title washed car better than most. In 2011, Zumhagen bought a used Dodge Neon with 75,000 miles on it from a dealership in Rockville, Maryland. Zumhagen says she test drove the car and allowed the dealer to have it inspected for her. When it seemed to be OK, she bought it for $3,500 cash -- the Blue Book value for the car.

Joanna Zumhagen, 23, with her used Dodge Neon, which was illegally repaired and resold after being totaled.

Within a few months, the problems started. Zumhagen, who is 23 and works as a marketing manager for a nonprofit, was driving on the interstate when she heard a whirring noise. A few minutes later, she said, her car's timing belt snapped, causing the power steering to fail.

Zumhagen stopped in the middle of an off-ramp and had the sedan towed to a shop, where a mechanic opened the hood to find that the engine was literally falling apart. In some places, Zumhagen said, it had been held together with plastic zip ties.

"And as you might imagine, some of them had melted, so part of the engine was actually wobbly loose on one side as a result," she said.

It wasn't until Zumhagen obtained a vehicle history report for her car that she found out the truth about its past: It had been totaled in 2009 in Maryland, issued a "salvage" brand and auctioned off by the insurance company. A car branded as a "salvage" vehicle means it's been wrecked to the point where an insurance company decides it doesn't make sense to pay to repair it. In most states, these vehicles can be legally repaired and resold, but sellers must always disclose that it's a salvage car.

According to the vehicle history report for Zumhagen's car, which was obtained by HuffPost, it was brought to Virginia, where it was issued a clean title. Two years later, it was sold to Zumhagen.

Virginia Department of Motor Vehicles spokeswoman Pam Goheen said the agency made an "oversight" in giving the car a title without a brand. Zumhagen bought the vehicle from SK Certified Auto Sales in Rockville, and she said no mention was made of its problematic past. SK Certified Sales declined to comment for this article. It is a crime to knowingly sell a vehicle that's been title washed.

"Basically, the engine had been jimmy-rigged back to working condition to make the car seem like it was OK," Zumhagen said. It's not clear why the mechanic who performed the inspection for the dealer didn't note the problems with the car, she said.

What happened to Zumhagen isn't all that uncommon. According to a report from Experian AutoCheck, which (like CarFax) sells vehicle history reports to consumers, 425,000 of the vehicles damaged in the first half of 2012 were later retitled as clean in another state.

Of course, for-profit businesses like Experian and CarFax have an incentive to scare people into thinking that many used cars have sordid histories concealed by new titles. Because if that's true, more people will pay $