A four-year investigation into the possible manipulation of the the silver market looks increasingly likely to be dropped after US regulators failed to find enough evidence to support a legal case, according to three people familiar with the situation.

The Commodity Futures Trading Commission first announced that it was investigating “complaints of misconduct in the silver market” in September 2008, following a barrage of allegations of manipulation from a group of precious metals investors.

In 2010, Bart Chilton, a CFTC commissioner, said that he believed there had been “fraudulent efforts” to “deviously control” the silver price.

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