The sight of a prim, uniformed German flight attendant struggling to lug her impossibly heavy hand luggage through the "nothing to declare" channel at Frankfurt airport finally convinced customs officials that something was up.

For the officers who ordered the luckless cabin crew member to open up her valise, it felt like stumbling upon Treasure Island. The flight attendant's luggage was groaning with thousands of gold- and silver-coloured one- and two-euro coins.

"Pulling open the zip on the case was like winning the jackpot on a fruit machine," is how one of the officials remembered the find at the airport last summer. Yet they also noticed that many of the euro coins in the luggage were oddly shaped, defaced or even bent. Questioned, the flight attendant nevertheless protested her innocence: "It's money a friend of mine in China gave me to trade in. No bank there will accept this sort of cash," she insisted.

Yesterday, the curtain was lifted on the origins of the mysterious haul of bent euros after state prosecutors announced the arrest of six people. Among them were four ethnic Chinese and flight attendants employed by the airline Lufthansa. They are suspected of involvement in one of the biggest professionally organised euro scams since the single currency's introduction.

"The six are being investigated on suspicion of importing forged coins," Frankfurt state prosecutor Doris Möller-Scheu said. She said a total of 25 people were thought to have belonged to the forgery ring. "Lufthansa has been informed that some of its employees are being investigated," she added.

Those in police custody in Frankfurt are accused of exploiting the Bundesbank's standard procedure for removing damaged or defaced euro coins from circulation and reissuing those who bring them to the bank with legitimate coinage or notes.

The six stand accused of re-importing €20m-worth of damaged and defaced coins which the Bundesbank believed it had sold off to China to be melted down as scrap metal. To minimise the risk of foul play, the Bundesbank deliberately dismantles damaged coins prior to their disposal. Little did its staff know that until earlier this year, the coins were being intercepted by a gang of forgers who arranged for them to be carefully reassembled by a team of accomplices in China.

The gang used airline cabin crew, especially those employed by Germany's normally reputable national carrier, Lufthansa, to act as "mules" and take the reassembled coins back to Germany. In return for sizeable payments, they dragged the coins in their hand luggage past what was assumed would be unsuspecting customs staff at Frankfurt international airport.