A U.S.-backed effort to bring Venezuelan gold into the American market has run into a major obstacle: refiners are wary of touching the metal because of concerns over its origins.

The New York Times reports that hundreds of millions of dollars’ worth of Venezuelan gold is now sitting in storage as refiners hesitate to process material that could be linked to criminal groups, corruption or environmental abuses.

The trade began earlier this year after Trafigura struck a deal with Venezuela’s state miner Minerven to supply gold for the U.S. market. U.S. authorities separately authorized transactions involving Venezuelan-origin gold as part of Washington’s push to reopen the country’s mining sector.

The Times says its investigation found that some gold entering Minerven’s supply chain may come from informal miners who make protection payments to the Tren de Guayana gang. One mining brigade leader told the newspaper that roughly 20% of his group’s production went to the gang, while another share was delivered to Minerven and could be mixed into gold destined for Trafigura.

Trafigura says its due diligence is continuing and that inspections have found no evidence of human-rights abuses, extortion, illegal taxation or financing of armed groups at the sites examined. The White House argues that bringing Western firms into Venezuela is part of an effort to clean up and formalize the country’s mining industry.