FRANKFURT, Germany - For German customs officers, the smell of money is best captured by their four-legged friends.

The airport in Frankfurt, the country's financial capital, has a trio of German shepherd dogs to help trace large sums of cash brought into the country. In 2012, seven offenders were caught on average each day, and 28 apprehensions were made specifically using dogs. By June of this year, the canine patrol had nabbed 20 offenders, the customs office said.

Individuals crossing European Union borders must declare cash exceeding 10,000 euros ($13,068). With countries including Switzerland and Luxembourg pressured to loosen bank-secrecy laws and the German government buying up lists of tax dodgers, more money squirreled away abroad is flowing back home. Notices served on cash offenders have jumped 11-fold since 2000 to 2,489 last year, data provided by the customs office show.

"Your dog at home that can do some tricks is equivalent to a bicycle, while our dog is a Porsche," said Marc Behre, a spokesman for the Bremen customs office, which also has a shepherd dog trained for cash. "We use only the best."

The dogs specialize in detecting euro and dollar notes. Animals are typically purchased at the age of one, undergo health checks and then receive an 18-month training routine. Robustness, size, life expectancy and temper determine which breeds are suitable, said Uwe Wittenberg, who heads the dog squad at Frankfurt airport.

German retail deposits have climbed in 22 of the past 24 months, according to data compiled by the European Central Bank. Deposits reached 1.8 trillion euros ($2.35 trillion) in May, an increase of 3 percent in the past year and up by more than 40 percent in the past decade. German three-month government bill rates are minus 0.02 percent, down from a high for the year of 0.05 percent at the end of January.

Paper to print currencies is made from cotton and linen, rather than wood pulp, and euro notes are all-cotton. The sniffer dogs probably react to the ink used to print on the notes, said Behre, though central banks keep the exact composition of their legal tender a secret.

More than 10 specially trained money-detecting dogs are used in Germany, with as many as four employed in Frankfurt, according to Bernd Roessler, a spokesman for the federal finance office. While dogs have been used to sniff for drugs for more than 40 years, training them to seek cash carried is more complex and has only existed since 2010.