© Reuters. A copy of bitcoin standing on PC motherboard is seen in this illustration picture

By Nate Raymond

(Reuters) - A former U.S. Secret Service agent sentenced to nearly six years in prison for stealing bitcoins during a probe into the online drug marketplace Silk Road was sentenced on Tuesday to an additional two years for another digital currency theft.

Shaun Bridges, 35, was sentenced by U.S. District Court Judge Richard Seeborg in San Francisco after pleading guilty in August to money laundering in the second criminal case to be brought against the former agent, prosecutors said.

Bridges, who served in the Secret Service's Baltimore field office, was sentenced in 2015 to 71 months in prison for diverting to his personal account over $800,000 worth of bitcoins during the Silk Road probe.

Before serving that sentence, though, Bridges was arrested again on new charges related to his theft of bitcoins that were at the time worth $359,005 but today are valued at $11.3 million, according to the industry publication CoinDesk.

A lawyer for Bridges declined to comment.

Silk Road operated for more than two years until it was shut down in October 2013, generating more than $214 million in sales of drugs and other illicit goods using bitcoins, prosecutors said.

Ross Ulbricht, who authorities say ran Silk Road under the alias "Dread Pirate Roberts," was sentenced in 2015 to life in prison after a federal jury in New York found him guilty of charges including distributing drugs through the internet.

Bridges was initially arrested in March 2015 along with another member of the Baltimore-based federal task force that investigated Silk Road, former U.S. Drug Enforcement Administration agent Carl Force.

Force later pleaded guilty to charges of extortion, money laundering and obstruction of justice for stealing bitcoins during the probe and for secretly soliciting payment from Ulbricht. He was sentenced in 2015 to 6-1/2 years in prison.

Prosecutors said the bitcoins at issue in the latest case against Bridges had been seized in 2014 from a digital currency exchange.

Prosecutors said that after signing a plea deal in the 2015 case, Bridges used his access to a so-called digital wallet containing the bitcoins that the government controlled to transfer the bitcoins to digital wallets he controlled.