Updated 07/30/12 – 4:22 p.m.

CHICAGO (CBS) — Investigators say they have cracked the case of an auto theft ring that stole some 200 vehicles in Chicago.

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The theft ring is accused of stealing BMWs, Mercedes Benzes, Land Rovers and Cadillacs. In all, 21 people were arrested for stealing hundreds of vehicles worth more than $4 million in aggregate.

Cook County State’s Attorney Anita Alvarez and Illinois Secretary of State Jesse White held a news conference on the charges Monday, and brought a shiny black Cadillac Escalade that had been among the stolen.

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The thieves stole vehicles that were under lease, changed the vehicle identification numbers (VIN), obtained fraudulent title applications, submitted them for Illinois titles, and finally sold the vehicles for cash.

The Secretary of State’s office first noticed the scheme when a series of fraudulent applications were submitted to the office, and they were traced back to lease vehicles that had been stolen. Some vehicles worth $50,000 were being sold for just $10,000.

Alvarez said auto theft is becoming more sophisticated and organized.

“While it may not be perceived as the most heinous of crimes, auto theft has increasingly become a sophisticated part of organized crime that involves not just the theft of a vehicle, but other types of frauds such as identity theft, forgery and loan fraud. As evidenced by this operation, law enforcement has coordinated and dedicated resources to attack this crime.”

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CBS 2’s Marissa Bailey reports investigators said the thieves exploited a loophole to steal luxury cars from multiple dealerships undetected.

“Originally, some of these guys were transporters – licensed in the state of Illinois. They had Illinois dealer licenses, so they had access to different databases would allow them to shop cars around, and give them free access to roam among the dealerships,” said Illinois State Police Sgt. Tim Gainer.

The thieves would show up to dealerships, telling them they had to transport certain cars back to the manufacturer. Instead, police said the thieves would cover up those cars’ VIN tags with a different VIN for a vehicle of the same type, which they had obtained from an online database. They’d also use the vehicle identification numbers they found online to create fake titles for the stolen cars.

Police said the thieves would then sell the stolen cars for thousands less than what they were worth.

“They’d put them up [on] Craigslist, newspapers, word on the street – any way they could get rid of a car,” Gainer said. “Most of the people that we were taking these vehicles from were other criminals, that were buying these cars for cash on the street.”

Authorities are also urging drivers to be thorough before buying used cars. Officials say buyers should make sure a car dealership is licensed by the Secretary of State’s office, check the vehicle identification number online, look up a history report for the vehicle, and remove the license plates.

White adds that if a deal on a used vehicles seems too good to be true, it probably is.

One major player in the theft ring, Dominique Henley, 23, of Homewood, has already pleaded guilty in DuPage County and has been sentenced to eight years in prison.

Henley’s father has also pleaded guilty and has been sentenced to 30 months’ probation.

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The other cases are pending.