Just an unusually heavy carry-on bag, which a female flight attendant struggled to carry through Frankfurt’s airport after arriving from China.

But when curious customs agents took a look, they found evidence that may have helped them crack one of the biggest frauds in history against the Bundesbank, Germany’s august central bank. Authorities say the fraud has cost the bank about 6 million euros, or $8.5 million.

Inside the flight attendant’s bag, the authorities say, were thousands of 1-euro and 2-euro coins that had supposedly been scrapped after years of use but had been methodically reconstructed so they could be cashed in.

According to prosecutors, in recent years a fraud ring involving flight attendants has toted 30 tons of supposedly scrapped euro coins back into Europe from China. Recyclers in China were supposed to have melted down the old coins, which had been removed from circulation and sold as scrap metal.

Instead, officials say, the band and its accomplices painstakingly restored the coins, then fooled the Bundesbank into redeeming them for paper currency or money transfers into bank accounts.

Last week, German authorities arrested six men in and around Frankfurt in connection with the case. The investigation is continuing, said Doris Möller-Scheu, a Frankfurt prosecutor.

No Bundesbank employees are suspected in the fraud, authorities said. But the case is an embarrassment for the central bank, long a symbol of German prudence and monetary stability.

Three of the six men arrested were flight attendants who worked for airlines with routes between China and Germany. The attendants took advantage of their exemption from baggage weight requirements to carry the coins from China to Frankfurt, prosecutors said.

The female flight attendant, who authorities said was not arrested because she had not redeemed any money, told them that Chinese friends had given her the coins to exchange abroad because domestic banks would not take them, according to Sing Tao, a Hong Kong newspaper.

All six suspects, ages 28 to 45, are being held awaiting trial, and so far none are cooperating with authorities, Ms. Möller-Scheu said. They face fraud and counterfeiting charges that carry maximum sentences of 10 years in prison. Under German law, the names of suspects are typically not released.

The fraud ring, which officials said had been operating since 2007, took advantage of the way the two-piece coins are made. The 2-euro coin has a nickel and brass alloy center, or “pill,” surrounded by a nickel and copper ring. The 1-euro coin has a copper and nickel pill surrounded by a nickel and brass ring.

When the coins were removed from circulation, a subcontractor separated the rings from the pills before the metal was sold to Chinese recycling companies.

Somehow, the thieves discovered that they could put the pills and rings back together, bring them to Germany and redeem them at the Bundesbank.

Prosecutors said they were just beginning to investigate how the scam worked in China. During raids of 10 buildings in and near Frankfurt on March 30, the police seized a machine that they think was used to reassemble coins. But Ms. Möller-Scheu said it was likely that most of the labor-intensive work of reassembling coins was done in China.

“It wouldn’t pay with the hourly wage here,” Ms. Möller-Scheu said. “It had to be a low-wage country.”

How does someone inconspicuously redeem tons of reassembled coins? It was not as hard as it might seem, the Bundesbank says. Chinese companies recycle enormous amounts of washing machines, autos and other worn-out goods from Europe, often finding euros, which they then send back for redemption.

“It wasn’t so unusual to get coins from China,” said Susanne Kreutzer, a Bundesbank spokeswoman. “That is a business model for some people.”

The thieves packed the coins in official-looking safe bags, which are used to trade coins for cash and are easily available on the Internet, prosecutors said. Some genuine coins were mixed in to fool inspectors, who make spot checks of redeemed coins.

The accomplices could turn in the coins at any of the Bundesbank’s 47 branches in Germany, although large amounts would be accepted only at the branch in Mainz, about 40 kilometers, or 25 miles, west of Frankfurt.

Ms. Kreutzer said there had been other occasions when the Bundesbank suspected that people were redeeming invalid coins, but last week was the first time authorities could make an arrest.

One question raised is why the coins were not more thoroughly damaged when they were taken out of circulation.

It is not clear which of Europe’s central banks might have been responsible for the coins involved in the Bundesbank scam. The Bundesbank said it could not have been the source of the coins, because it rendered old euro pieces unusable by crimping them with deep ridges.

Ms. Möller-Scheu said there was no evidence to support a report in the news magazine Der Spiegel that the coins had come from Italy and Greece.

In any case, Bundesbank officials said they did not think the scheme could be carried out in the future. In January, new European Union rules took effect placing tighter restrictions on the redemption of coins.

“This business model won’t work any more,” said Ms. Kreutzer of the Bundesbank.

Jon Kaiman contributed research from Beijing.

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