The U.S. Justice Department announced on November 5 it has seized nearly 70,000 Bitcoins from a person the agency would describe only as "Individual X."

The news is remarkable because of the value of Bitcoin seized—the haul is worth around $1.05 billion based on today's Bitcoin price of $15,000—but also because of the timing and the mysterious nature of the seizure.

According to a Justice Department complaint, the digital wallet holding the Bitcoins belonged to a hacker who stole them from the operator of the Silk Road, a notorious black market website that acted as a giant online bazaar for drugs and other criminal activity. The federal government took down the Silk Road and arrested its owner—who is now serving life in prison—in 2013.

The Justice Department complaint says IRS agents reviewed the hacker's activities earlier this year with the help of a cryptocurrency forensics firm. That firm, Chainalysis, published a blog post detailing how it analyzed the Bitcoin blockchain—a tamper-proof public ledger of transactions—to track the hacker's activities.

The blog post includes the graphic below, which shows the stolen Bitcoins moving to different wallets, including one transaction in which the hacker moved 101 Bitcoins to a now-shuttered criminal exchange called BTC-e:

According to a person familiar with the investigation, the hacker's robbery of Silk Road—which took the form of 54 transactions in less than 24 hours—took place in 2012. Meanwhile, the transfer of 101 Bitcoins took place in 2015, with no further transactions after that.

This raises the question of why the so-called Individual X hasn't touched any of the funds, even as Bitcoin has soared in price. There appear to be two possible explanations: The person is a wealthy individual who did not need to sell any of the Bitcoins, or else the person is in prison without access to a website needed to transfer the funds.

The source familiar with the investigation said it is significant the complaint was filed in San Francisco, and that Individual X signed a consent decree. This implies the person in question is located in Northern California and is cooperating with law enforcement—as does the fact the Justice Department was able to seize the Bitcoin, which could only have occurred if the person provided the password or "private key" needed to transfer Bitcoin funds.

As for why the Justice Department didn't name the individual, the source speculated it was because the person could be at risk of violent retaliation from criminals tied to the Silk Road.

“They’re not in the safest business in the world," the source said.

The Justice Department did not immediately respond to a request for comment as to whether Individual X is in prison. Nor did the agency state if a criminal complaint—as opposed to the civil one unveiled—will be forthcoming. The lack of any criminal complaint may suggest Individual X is already incarcerated and may have agreed to turn over the Bitcoin as part of a cooperation arrangement with authorities.

The Justice Department seized not only 69,370.22491543 Bitcoins from Individual X, but the same amount in three spinoff currencies known as Bitcoin Cash, Bitcoin Gold, and Bitcoin SV. Together, those other currencies are worth around $30 million.

The government hasn't announced what it will do with the seized Bitcoins, but in the past it has sold them though auctions run by the U.S. Marshals Service, with the proceeds primarily going to law enforcement agencies.