The accused mastermind of the online black market website Silk Road, arrested this week, has been charged in a murder-for-hire plot, court documents show.

The indictment unsealed Wednesday in federal court in Maryland charges Ross William Ulbricht, also known as "Dread Pirate Roberts," with paying an undercover federal agent $80,000 to kill a drug buyer Ulbricht feared would reveal details of his criminal enterprise.

The indictment also charges Ulbricht with conspiracy to distribute drugs, witness tampering and other criminal acts.

The formal charges were unsealed following Ulbricht's arrest in San Francisco on Wednesday after a lengthy FBI investigation into the online for drugs, hitmen, hacker tools and more.

Federal agents shut down the website, which used a privacy-protecting Tor network and Bitcoin digital currency to shield the identities of buyers and sellers around the world.

According to the indictment, Ulbricht agreed to a series of drug sales with the undercover agent, and later indicated he was concerned that one of his employees had stolen funds from Silk Road and had later been arrested.

Ulbricht said he wanted the man beaten up, then changed his mind and asked for an "order to execute."

The document said Ulbricht transferred $40,000 from an Australian bank account as a downpayment and another $40,000 after "proof" of death, which the indictment said was a staged photo.

Ulbricht was set to appear in a California court Friday, when a judge will decide the next steps to take. A on his arrest was unsealed on Wednesday.

Ulbricht, 29, anonymized Silk Road transactions by using a Tor computer network designed to make it almost impossible to locate computers used to host or access websites.

He also added a Bitcoin "tumbler" to the Silk Road payment system to foil efforts to trace digital currency back to buyers, according to the criminal complaint.

Prosecutors maintained that Silk Road has been used by thousands of drug dealers to distribute hundreds of kilograms of illegal wares to more than 100,000 buyers and to launder hundreds of millions of dollars in ill-gotten profits.