Gold, copper, crude oil and other commodities tumbled on Wednesday amid worries that the Federal Reserve may pull the plug on its stimulus measures—and speculation that a big hedge fund was selling.

Gold fell as much as 2.9% in after-hours trading, the steepest drop in almost a year, after minutes from the Fed's latest meeting outlined more debate about when to curtail the central bank's bond-buying program. Meanwhile, crude oil settled 2.3% lower, with most of the declines coming in a frantic, 10-minute selloff after a surge...