U.S. commodity regulators have closed a high-profile investigation into alleged manipulation of the silver market without charging any firms or individuals, concluding there is no “viable basis” for a case.

The decision by the Commodity Futures Trading Commission comes five years after the case was opened and is likely to disappoint some silver traders, market observers and regulators. Some contend that silver prices have been artificially depressed by financial institutions seeking to profit by betting against the market.