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A former Secret Service special agent who helped bring down Silk Road founder Ross Ulbricht will plead guilty to skimming hundreds of thousands of dollars in bitcoin and routing the money to his own accounts, according to federal court documents.

Shaun W. Bridges, a computer crime expert, was part of the Secret Service task force that dug up the evidence that led to charges against Ulbricht and the shutdown in 2013 of Silk Road, the underground online black market he ran. Ulbricht eventually was convicted of narcotics trafficking, computer hacking, running a criminal enterprise and money laundering and was sentenced to life in prison last month.

But Bridges dug up more than just evidence, according to a criminal complaint filed in U.S. District Court in San Francisco.

The drug and weapons black market Silk Road was shut down in the federal investigation in 2013. NBC NEWS

Using information he got from his interrogation of a Silk Road customer service representative who had administrator access, Bridges accessed Silk Road's systems in January 2013, reset several passwords and transferred about 20,000 bitcoin from the accounts, according to the complaint. He then converted the bitcoin into cash through a Japan-based bitcoin exchange and from there sent the money to the shell company's investment accounts, investigators said.

The value of bitcoin in U.S. dollars fluctuates significantly, making it impossible to say what the haul was definitively "worth." But based on conversion rates at the time various transactions were made, federal prosecutors put the value of Bridges' loot at $820,000.

In court documents filed last week, Bridges agreed to plead guilty to wire fraud and money laundering and is scheduled for sentencing Aug. 31.

A second investigator, Carl Force, the Drug Enforcement Administration's lead undercover agent in the Silk Road case, faces a hearing in August for allegedly stealing bitcoin from Silk Road — and for allegedly blackmailing Ulbricht while at the same time selling him information about the investigation.