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.

But after taking advice from two external consultancies, the first of which found irregularities on certain trading dates that it believed deserved more analysis, CFTC staff do not have sufficient evidence to bring a case, according to the people familiar with the situation.

The agency’s five commissioners have not yet formally determined the outcome of the investigation, leaving the possibility that staff could be instructed to dig deeper. A CFTC spokesman said: “The investigation has not reached its conclusion”. He declined further comment.

Ending the probe would infuriate some US silver investors, who claim that a group of large investment banks – in particular, JPMorgan – has conspired to drive the price of silver lower.

“I’m sure it will be met with some concern from a certain group of aggressive silver speculators,” said one person familiar with the investigation.

In a recent blog post, Ted Butler, a newsletter publisher and unofficial champion for the silver investors, accused the CFTC of being “negligent in failing to terminate theobvious manipulation ongoing in silver”.

The CFTC has analysed over 100,000 documents and interviewed dozens of witnesses since it began investigating the market in 2008, it said last year. The people familiar with the situation said the evidence included records from JPMorgan.

The conclusion of the investigation will come as a relief to JPMorgan. Although no company or individual was named in the CFTC investigation, the Wall Street bank has suffered a torrent of allegations from silver investors on the blogosphere.

One campaign exhorted sympathetic readers to “crash JPMorgan” by buying silver – based on the assumption, which JPMorgan has repeatedly denied, that the US bank has a large bet on lower silver prices.

In addition, a class-action lawsuit has been filed against JPMorgan. Lawyers for the bank have asked a judge to dismiss it, arguing that plaintiffs “fail to identify a single trade” showing manipulation.

Blythe Masters, head of commodities at JPMorgan, in an April interview with CNBC conceded that there had been “a tremendous amount of speculation, particularly in the blogosphere, about this topic”, but maintained that the bank had no large bets on silver prices.

“We have no stake in whether prices rise or decline,” she said. JPMorgan declined to comment on the CFTC investigation.

Previous CFTC silver inquiries in 2004 and 2008 found no evidence of wrongdoing.