F.B.I. Seizes Silk Road, an Online Drug Market, and Makes Arrest Authorities have posted this notice on the Silk Road Web site, which the F.B.I. seized. Federal authorities have charged a man with running Silk Road, a popular online black market for drugs. The authorities identified the man as Ross Ulbricht, who was arrested by F.B.I. agents Tuesday afternoon at a library in San Francisco. Court papers filed in the case in Manhattan accuse him of engaging in a "massive money-laundering" operation and of trying to arrange a murder-for-hire. Mr. Ulbricht is to appear in federal court on Wednesday in San Francisco. Mr. Ulbricht solicited a Silk Road user "to execute a murder-for-hire of another Silk Road user, who was threatening to release the identities of thousands of users of the site," according to a criminal complaint unsealed Wednesday morning. Authorities also seized the Silk Road Web site. In interviews with news reporters in the past, Mr. Ulbricht has gone by the moniker Dread Pirate Roberts, the authorities said, and until now his identity has remained a mystery. The Silk Road marketplace is available through Tor, a popular tool for maintaining anonymity online. Bitcoin, a virtual currency, is used for transactions. The identities of sellers are not known to the buyers. About $1.2 million in sales were conducted a month in early 2012, according to a study by an assistant professor at Carnegie Mellon University, Nicolas Christin. As part of the investigation into Silk Road, authorities said, they seized 26,000 bitcoins worth $3.6 million. The arrest is part of the latest push by federal authorities to police the anonymous marketplaces that have flourished as a result of virtual currencies and software meant to help users browse the Web anonymously. In recent months, federal authorities charged seven people believed to be linked to Liberty Reserve, another virtual currency, which prosecutors described as a $6 billion money-laundering operation that facilitated a black market for everything from stolen identities to child pornography. One recent study found that a broad range of drugs, including ecstasy, LSD and heroin, were available on Silk Road, but that marijuana was the most popular item offered for sale. Books and erotica are also sold.