A federal judge in Manhattan denied bail Thursday to Ross Ulbricht after federal prosecutors alleged that he plotted six killings earlier this year to protect his position as the operator of the sprawling online drug market Silk Road.

Assistant U.S. Attorney Serrin Turner said it's not clear whether five of the intended victims actually exist. But Turner argued that Ulbricht could not be released without endangering the public or running the risk that he would flee. He said the 29-year-old had explored obtaining citizenship in another country and ordered fake identity documents.

Two murder-for-hire plots that authorities say Ulbricht put into motion were detailed in the charges against him unsealed in October. The four new attempts are described in a filing federal prosecutors made to support their request that he be held without bail.

Ulbricht is accused of running Silk Road over the course of more than two years and building it into the most popular place to buy drugs online. Buyers and sellers used sophisticated technology to hide their identities, but authorities say they were able to unmask Ulbricht through a combination of undercover work and online sleuthing.

Ulbricht, who allegedly went by the name Dread Pirate Roberts, is accused of hiring a user known as redandwhite — a reference to the Hells Angels, according to the government — to kill five people he believed were living in Canada.

The FBI and Canadian authorities could find no evidence that the targets actually exist. Nevertheless, federal prosecutors argued that Ulbricht's apparent willingness to pay for killings meant he should be held in jail.

"Ulbricht should be detained as a danger to the community," Turner wrote. "Given his willingness to pay approximately $730,000 for attempts to kill six people, there is no reason to believe that he would not again resort to violence."

Joshua L. Dratel, Ulbricht's attorney, argued that his client had a strong network of friends and family who could vouch for him and proposed a bail package of $1 million to assure Ulbricht's appearance in court.

But Turner wrote that Ulbricht was skilled at leading a double life, even with the people closest to him.

"While portraying himself as a champion of 'freedom' on the Silk Road website, opposed to the use of any kind of 'force' against others, he was in fact a quite ruthless criminal," Turner wrote, "one who, with seeming ease and lack of conscience, nonchalantly ordered murders for hire amidst fixing server bugs and answering customer support tickets."

Reuters contributed to this article.

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