(Reuters) — U.S. prosecutors have concluded that a former Wyoming Retirement System chief investment officer’s insider trading guilty plea is no longer valid following a major appellate ruling limiting the ability of authorities to prosecute insider trading cases.

Prosecutors in a letter filed on Friday in Manhattan federal court said they believed admissions John Johnson made in 2013 about his trading ahead of a technology merger were “insufficient to support the guilty plea” following the ruling.

But the prosecutors under Manhattan U.S. Attorney Preet Bharara said they were still prepared to take Mr. Johnson to trial if he does not enter a new plea.

Mr. Johnson’s lawyer did not immediately respond to a request for comment Monday.

The letter marked the latest fallout from a December ruling by the 2nd U.S. Circuit Court of Appeals in New York that curtailed authorities’ ability to crack down on insider trading.

The court, in reversing the convictions of two hedge fund managers, Todd Newman and Anthony Chiasson, ruled that prosecutors must prove a trader knew the source of a tip received a benefit in exchange for the information.

The court also narrowed what constitutes a benefit for the purposes of insider trading prosecutions, saying it must be of “some consequence” and cannot be only friendship.

Prosecutors are appealing the decision, which has led to convictions being reversed or charges being dropped for seven of the 93 insider trading defendants Mr. Bharara’s office had pursued since 2009.

Mr. Johnson pleaded guilty to securities fraud and conspiracy charges. He later testified in the insider trading trial of David Riley, former chief information officer of computer equipment company Foundry Networks Inc, who was convicted in October.

Prosecutors said Mr. Riley in 2008 told Matthew Teeple, then an analyst at hedge fund Artis Capital Management, about Brocade Communications Systems Inc.’s proposed $3 billion acquisition of Foundry.

Mr. Riley told others, prosecutors said, including Mr. Johnson, who testified that, before taking the Wyoming pension system job, he traded in Foundry stock based on the tip.

But Mr. Johnson never said he knew Mr. Teeple’s source, nor did he acknowledge knowing Mr. Riley received anything for the tip. Last month, U.S. District Judge Valerie Caproni questioned the sufficiency of the plea.

Prosecutors on Friday said if they took Mr. Johnson to trial, they could still prove “he consciously avoided knowledge of the tipper’s benefit.”

Mr. Teeple pleaded guilty and was sentenced in October to five years in prison.