FORST, Germany—Returning home from a midnight wild-boar hunt near his village along the Polish frontier in July, German farmer Matthias Rattei met an unexpected group of visitors: thieves trying to steal a €250,000 harvester and two tractors.

After several years of decline, thefts of luxury cars and heavy duty equipment such as tractors and trucks are surging along Germany's eastern border. In 2009, vehicle thefts in four of the five German states near the eastern frontier with Poland and the Czech Republic rose to their highest levels since 2000. In Berlin, a 45-minute drive to the Polish border, thefts jumped more than 40%, representing more than €40 million (about $50 million) in lost and damaged vehicles, insurance analysts say.

To many in this remote stretch of eastern Germany, the reason for the increase in vehicle larceny is clear: the elimination of border controls in 2007.

The lifting of the controls was widely seen as a milestone for European integration. It was particularly poignant in the relationship between Berlin and Warsaw. Large swaths of modern Poland previously belonged to Germany, and the border opening signaled that both were prepared to put aside animosities that long divided the two states.

As part of the so-called Schengen Agreement, travelers can pass borders freely across much of Europe. Yet for all of the benefits the change has brought, political and economic, German authorities say the move has unleashed a wave of thefts that is testing the limits of European Union law and overwhelming their resources.

Forces behind the crime—black market demand for high-quality cars and machinery in Russia, and depressed stretches of Poland with young men willing to break the law to secure them—are unlikely to change anytime soon, they say.

"It's not a short-term phenomenon, and we shouldn't react in a short-term way," says Arne Feuring, the police chief in Brandenburg, a state of shrinking towns and flat farmland that surrounds Berlin and borders Poland.

Every night, criminals from Poland and further east slip into Germany, steal luxury Mercedes or farm combines, and sneak back, police officials say. The thieves deliver stolen vehicles to overseers who strip them of identifying features for resale in Ukraine or Russia, police say.

In July, German police scrambled a helicopter and four squad cars to retrieve the private Audi sedan of Interior Minister Thomas de Maiziére—who oversees Germany's federal police force. The suspected car thief, a 33-year-old Pole, was heading toward the border when police forced him to pull over.

Organized Eastern European criminals have been stealing cars in western Europe since the 1990s, but German authorities say that along their open border the thieves have become increasingly brazen and pursue higher-ticket items, especially farm and construction machinery. The criminals communicate over short-wave radio and tote laptops to start computer-protected engines.

Mr. Feuring, the Brandenburg police chief, says countries with open borders will need to adjust their penal codes to handle criminals who can move between nations unimpeded. "It's not just a German problem, or a German-Polish problem," he says. "It's a European problem."

For now, German federal and state police have focused on improving cooperation with their Polish counterparts. In a concrete former border checkpoint in the Polish town of Swiecko, teams of German and Polish police and customs officials face off across two rows of computer monitors 24 hours a day, handling 14,000 requests for cross-border assistance a year.

A focus is connecting German and Polish police officers during stolen vehicle operations. But they've failed to reduce thefts. They say the criminals are usually faster than the police.

"When you're on patrol 20 kilometers from the border and you determine that a car is stolen, and the driver hits the gas while you call the cooperative center, and then the center calls the Polish police—by then he's all the way to Ukraine," says Heiko Teggatz, a federal police officer based in Brandenburg who is active in one of Germany's major police unions.

Mr. Rattei's father, Egon, agrees; he manages a 5,500-acre crop, cattle and dairy farm nestled between Forst and the border. "They're highly organized, and very professional," says the elder Mr. Rattei, who lost a tractor to thieves in April.

German police were on hand to help him and his son in July, but it was good fortune and quick thinking that spared his farm equipment, he says.

After getting the call from his son that their farm equipment was heading toward the border, Mr. Rattei, who had been asleep, hopped in his car without changing out of his pajamas.

With the police and the Ratteis in hot pursuit, the tractor thieves ditched the moving equipment close to the border and disappeared into the woods. Polish authorities found no trace of them.

Earlier this month, someone broke into five of Mr. Rattei's tractors late at night but failed to get them started.

"Every night as I'm falling asleep, I think, "Is something happening right now?'" he said. "It has a real impact on your psyche."

Write to Patrick McGroarty at patrick.mcgroarty@dowjones.com