tight control over sales to domestic dealers and to foreign countries.

Last year, federal prosecutors and agents with the Secret Service and the Department of Homeland Security began a broad crackdown on this “gray market” export business, which is estimated by some to be responsible for sending as many as 35,000 new luxury cars a year to China from the United States.

Federal prosecutors in half a dozen states — New Hampshire, New Jersey, Ohio, New York, Texas and South Carolina — have filed criminal or civil actions seeking to put a halt to the resale of luxury cars to China. Prosecutors have frozen bank accounts containing the proceeds from auto sales and seized hundreds of cars, some waiting to be shipped from cargo ports in Newark, Staten Island andintend to ship the newly purchased cars overseas.

Federal authorities are moving forward with illegal export cases at a time when luxury auto manufacturers are pressing to take advantage of the demand for high-end cars in China. On Wednesday, Daimler, the parent company of Mercedes, said it had sold 24,199 cars in China in January, an increase of 44 percent from a year ago.

So far, most of the actions brought have been civil in nature, as the New Hampshire action is the only one in which anyone pleaded guilty to a crime.

But that may change as the New York attorney general, Eric T. Schneiderman, has opened a criminal investigation into activity involving luxury car dealerships in New York and New Jersey, people briefed on the matter said.

Lawyers, including former