This could be a coincidence. It could also be a coincidence that neither man nor pseudonym seemed motivated by greed. Mr. Ulbricht’s lifestyle was one notch above that of urban couch surfer. And the primary goal that D.P.R. professed was to unshackle humanity from what he regarded as economic tyranny. If a handful of miscreants — and yes, a few of their unfortunate roommates — were killed along the way, that is a shame. But Silk Road was like a tunnel under the gulag, and D.P.R. was digging for the sake of humanity.

“It needs everything we have,” D.P.R. wrote of Silk Road, urging buyers and sellers to grasp its significance. “Do it for me, do it for yourself, do it for your families and friends, and do it for mankind.”

Perhaps this is where Dread Pirate Roberts, the criminal mastermind, overlaps with Ross Ulbricht, the guy who aided a homeless woman in a wheelchair. With aching sincerity, they both really wanted to help.

Roads Closed?

The success of Silk Road raised the prospect that the thuggish street drug dealer, the stock character of countless police procedurals, could be replaced by a geek in a coffee shop with a laptop. But in the months since Silk Road was shut down, several sites have tried to takes its place and all have failed.

One of them, called the Sheep Marketplace, went dark in December, as users howled that administrators had made off with as much as $44 million in Bitcoins. Project: Black Flag lasted a few weeks in October, before its leader announced, “I was unable to cope with the stress and constant demand, so I panicked.” Metta Dread Pirate Roberts, as that site’s fearful leader was known, then absconded with an unknown sum of Bitcoins, according to news reports.

“I suspect that the online drug marketplace is a passing fad because it’s too traceable, too vulnerable to hacking,” said Mr. Weaver at the International Computer Science Institute. Once Bitcoins are converted to another currency, the government can subpoena the records of the exchange where the transaction took place and harvest all the information it needs. “Bitcoin isn’t really a ‘coin’ as much as a distributed, public balance ledger,” Mr. Weaver added, “with every balance and transaction recorded.”

Hence Bitcoin’s wry new nickname in legal circles: “Prosecution Futures.” The government has begun making arrests: Olivia Bolles, a Delaware doctor, was charged in November with selling oxycodone, Xanax and other drugs on Silk Road, stuffing Sour Patch and Jolly Rancher candy in the packages to thwart detection.

But the limits of technology are only part of the reason that another Silk Road is unlikely anytime soon. To function, such a site needs a leader who is dedicated to the point of fanaticism, and, more important, has a strange kind of integrity. Dread Pirate Roberts did not take the Bitcoin and run because he was a true believer first and an outlaw second. He was a rare set of contradictions, a humanitarian willing to kill, a criminal with a strict code of ethics.