A Commodity Futures Trading Commission regulator is putting pressure on the agency to take action in a high-profile, two-year-old investigation of the silver market.

At a CFTC hearing Tuesday to consider new rules to strengthen its commodity-enforcement powers, commissioner Bart Chilton said market players have made "repeated" and "fraudulent efforts to persuade and deviously control" silver prices. Mr. Chilton said he believed there have been violations of CFTC rules that should be prosecuted, though he couldn't publicly...