Large banks and trading firms are frantically trying to determine whether they have fallen victim to a suspected commodities fraud emanating from the giant Qingdao Port in northeast China. Citigroup and several other large Western banks are concerned that their loans may lack the appropriate collateral, big stockpiles of copper and aluminum at the port. The banks have inspectors on the ground who are trying to assess whether enough of the metals are there. The worry stems from suspicions that a Chinese company pledged the same collateral for multiple loans. Chinese authorities are investigating the matter. The case could have broad repercussions for the commodities market and the Chinese economy. Banks have funneled billions of dollars into the Chinese economy through these murky transactions, and commodities prices have been falling over concerns that such lending will dry up. Western banks, including Citigroup, are bracing for any potential fallout. Just months ago, Citigroup fell victim to a multimillion-dollar fraud in Mexico. If the Qingdao developments harm the bank, regulators and shareholders are likely to press it to explain why its controls had failed again. Chinese companies are at risk, too. Citic Resources, part of the state-controlled conglomerate Citic Group, plunged nearly 10 percent on Tuesday after it disclosed that it might be affected by an investigation into stockpiles of metals held at the port. Citic Resources said on Monday that it had asked the local Chinese courts to secure its metals stockpiles. The shares recovered on Wednesday. The potential fraud is linked to an opaque corner of China?s financial system that has grown substantially in recent years, bringing huge amounts of capital into the country. Many Chinese companies and investors, struggling to secure traditional loans from the state-dominated banking sector, have instead turned to alternative, unregulated financing methods involving imports of materials like copper, aluminum and iron ore. These commodities financing deals are part of a growing number of nontraditional lending activities that have pushed credit in China to levels that are raising fears among investors and analysts. Jonathan Cornish, the head of North Asia bank ratings at Fitch Ratings, estimates that total outstanding credit in China rose to more than 220 percent of gross domestic product last year, up from 130 percent in 2008. A typical commodities financing deal works like this: Copper is imported using letters of credit, warehoused in duty-free zones and pledged as collateral for cheap bank loans. The loan proceeds are used by the importer to speculate in higher-yielding, short-term investments. The importer then either sells the commodity or the investment product after a few months when the original letter of credit falls due. The problem in Qingdao appears to revolve around one such importer. Last Friday, Qingdao Port International, the biggest port operator in the Chinese city, announced that the authorities had begun investigating a suspected fraud related to the aluminum and copper stored in its warehouses. A day earlier, a report in The 21st Century Business Herald, a respected Chinese-language newspaper, identified the company under investigation as Qingdao Decheng Mining. The report said Qingdao Decheng was suspected by the authorities of having pledged the same stocks of the metals ? about 100,000 tons of aluminum and 2,000 to 3,000 tons of copper ? as collateral for multiple loans, amassing bank debt exceeding 1 billion renminbi, or $160 million. Phone calls and emails to Qingdao Decheng?s parent company, Dezheng Resources, went unanswered on Wednesday.