When Alexandre Cazes hanged himself in a Thai jail cell in July, the 25-year-old left behind the trappings of a big-league drug dealer: villas, Lamborghinis, a Porsche, bank accounts in Liechtenstein and Switzerland. But Cazes, who authorities allege operated AlphaBay, the world’s largest black-market website for drugs and weapons, also left something else: Internet “wallets” holding millions of dollars’ worth of Bitcoin and other virtual currencies.

Cazes’s digital loot is now property of the U.S. Justice Department, which seized it during a global sting operation. The agency plans to sell it, and given that Bitcoin’s value has soared more than fivefold since then, it could reap a huge windfall. But if you want to find out who’s holding those coins, or when they’re being sold, you’ll need extensive cybersleuthing skills—and a lot of free time.

These digital seizures and sales, unheard-of five years ago, are fast becoming routine. Bitcoin’s enduring popularity among online wrongdoers, and its growing presence in criminal busts, has turned Uncle Sam into a major player in cryptocurrency markets. While exact figures are impossible to pin down, documentary evidence and interviews with current and former defense attorneys and prosecutors suggest that at least $1 billion worth of digital coins, and possibly much more, has spent time in the custody of U.S. law enforcement.

But once in government hands, this digital hoard disappears behind a cloak of secrecy. The anonymity that makes Bitcoin a darling of libertarians—along with opaque property-seizure laws hated by those same libertarians—makes it virtually impossible for the public to follow the digital money. And as federal agencies have been drawn into an ever-growing role in the cryptocurrency boom, their efforts to guard their digital gold have led to surprises, stumbles, and sometimes sin.

The U.S. Marshals Service is the oldest law-enforcement agency in the country, counting gunslingers like Wyatt Earp and Wild Bill Hickok among its alumni. More recently, TV and movies have familiarized many Americans with its role transporting prisoners and tracking dangerous fugitives. Far fewer people know the marshals sell Bitcoin.

A decades-old law gives the Marshals Service, which is part of the Department of Justice, primary responsibility for disposing of items seized by other federal law-enforcement agencies. That’s why you can visit the marshals’ website and ogle boats, cars, planes, wristwatches, and other ill-gotten gains snatched by the FBI and other agencies, all available at public auction. The seizure process, known as forfeiture (see sidebar), became more commonplace and controversial in the 1980s after Congress made it easier for federal officials to sell assets tied to drug crimes.

At the time, no one knew these assets would someday include money mined on computers. That changed earlier this decade during an epic investigation into Silk Road—a global eBay for illegal drugs. A young Texan known as Dread Pirate Roberts (real name: Ross Ulbricht) built Silk Road on three then-new technologies: cheap cloud data storage; the Tor browser, which let people roam dark parts of the Internet undetected; and Bitcoin, which let them pay each other in a secure, semi-anonymous manner, without involving banks.

去年7月，25岁的亚历山大·卡兹在泰国一家监狱自缢身亡，身后留下了堪比顶级毒贩的财物：别墅、兰博基尼、保时捷、列支敦士登和瑞士的银行账户。官方表示，卡兹是全球最大的毒品和武器黑市网站阿尔法湾（AlphaBay）的运营者。他还留下了另一样东西：装有价值数百万美元的比特币及其他虚拟货币的电子“钱包”。

美国司法部在一次全球突击行动中缴获了卡兹的虚拟财产，它们现在归该部所有，司法部计划将其出售。比特币的价格自缴获时到现在，已经猛涨了原来的5倍有余，司法部将获益丰厚。不过如果想要知道比特币的持有者是谁，或它何时被交易，必须具备强大的网络侦查技能，以及大量的空闲时间。

数字货币的收缴和销售，在五年前还鲜为人知，而今正快速成为普通事物。比特币长期以来深受网络罪犯喜爱，现在也越来越多地出现在刑事搜捕案件当中，它使得美国成为了加密货币市场的主要参与者。我们无法得知确切的数字，不过根据书面证据和对现任及前任辩护律师和检察官的采访，可以推算出美国执法部门至少保管着价值10亿美元的数字货币，并且实际金额很可能要远高于此。

数字货币一旦进入政府手中，便蒙上了神秘的面纱。比特币的匿名性让它广受自由论者喜爱，不过加上这些自由论者所痛恨的不透明的财产没收法律，公众几乎不可能追踪数字货币流向。联邦机构为加密货币的繁荣发挥着日趋重要的作用，他们为守护手中的数字黄金做出了令人讶异的举动，乃至犯下错误或者犯罪。

美国法警局（U.S. Marshals Service）是美国历史最为悠久的执法部门，枪手怀特·厄普和“狂野比尔”希科克都曾是它的成员。最近，电视和电影作品让许多美国人了解到它如何运送囚犯及追踪危险逃犯，不过很少有人知道，法警局还出售比特币。

根据拥有几十年历史的一条法律规定，法警局这个隶属于美国司法部的机构，有一项主要职责是处理其它联邦执法机构缴获的物品，这也是为什么访问法警局网站可以看到被联邦调查局和其他机构收缴的船只、汽车、飞机、手表等非法财物，这些物品全都参加公开拍卖。20世纪80年代，国会的一项决议让联邦官员能更容易地出售毒品犯罪相关财物，此后没收财物程序（见边栏）变得更为普遍，也更具争议性。

那时，没有人知道将来这些财产当中还会包括在电脑上挖矿获得的金钱。这种认知在前几年对丝绸之路（Silk Road）的大规模调查中就已产生改变。丝绸之路是一家类似eBay的全球性非法毒品销售网站。一位来自德克萨斯州的年轻人恐怖海盗罗伯茨（真名：罗斯·乌布利希），利用三项当时的新技术建造了丝绸之路网站。它们分别是廉价的云数据存储；允许用户隐秘游览网络世界黑暗深处的洋葱浏览器；以及允许用户不通过银行，以安全、半匿名方式相互支付的比特币。

By 2013, as the feds closed in on Silk Road, criminals had become savvy about Bitcoin, but law enforcement lagged behind. “There was no expertise. It was too new,” says a prosecutor involved in the case. Like most sophisticated Bitcoin users at the time (and most criminals today) the Dread Pirate didn’t rely on a broker such as Coinbase to hold his digital funds. Instead, Ulbricht controlled an online wallet using a private key—a long, complex set of characters that’s basically impossible to guess. In private-key cases, the only way law enforcement can quickly obtain the Bitcoin is if the suspect reveals the key.Enterprising agents figured out ways to snag suspects’ currency when it wasn’t protected. To get Ulbricht’s Bitcoin, they snatched his open, unlocked laptop from under his nose while arresting him in a San Francisco library. (As for Cazes, he was logged in to an AlphaBay administrator’s account when agents rammed a car through the gate of his Thailand estate.) By the time they busted the Dread Pirate, the marshals had got up to speed: They controlled at least two digital wallets of their own, to hold the Silk Road currency and receive Bitcoins seized by other agencies. “This was cutting-edge stuff,” says Sharon Cohen Levin, a longtime chief of the asset-forfeiture unit in the U.S. Attorney’s office for the Southern District of New York, and now a partner at WilmerHale. “We’d never done something like this.” Once they did, however, the marshals fell back on standard procedure, preparing to handle the Bitcoin the same way they would a coke smuggler’s speedboat: by auctioning it off. That posed challenges because of the sheer size of the seizure—about 175,000 Bitcoins, or 2% of all the Bitcoin in circulation at the time. According to a prosecutor familiar with the case, the marshals opted for a staggered series of auctions to avoid crashing Bitcoin’s price.In four auctions between June 2014 and November 2015, the marshals sold the Silk Road Bitcoins for an average price of $379. Bitcoin went on to enjoy a huge run-up; as a point of comparison, in an unrelated auction this January, the marshals sold off 3,813 Bitcoins and netted $45 million—or about $11,800 per coin.Sold at those prices, the Silk Road stash could have reaped $2.1 billion, enough to cover the Marshals Service’s annual budget; in 2014 and 2015, it netted just $66 million. Billionaire venture capitalist Tim Draper, meanwhile, made what might be the investment of the decade when he snapped up 30,000 of the Silk Road coins for about $600 each.Draper, who described the auction process to Fortune as “smooth,” says he hasn’t sold a single one, adding, “Why would I trade the future for the past?”

2013年联邦政府查封丝绸之路网站时，罪犯们已经精通比特币，但执法部门未及时跟上。参与该案的一位检察官说：“对此没有任何专业知识，它实在是太新了。”恐怖海盗跟当时（及现在的多数罪犯）大部分精明的比特币用户一样，并不依赖于Coinbase这样的中介来持有数字货币。乌布利希利用私人密钥——一长串几乎不可能破解的字符——来控制他的电子钱包。执法部门想要获取设有私人密钥的比特币，唯一的方式就是嫌犯泄露密钥信息。不甘放弃的特工们想出了在嫌犯货币未受保护时进行缴获的方法。为得到乌布利希的比特币，他们在旧金山图书馆逮捕他时，当他的面夺走他开着机且未上锁的笔记本电脑。（卡兹一案中，特工们开车迅速进入他在泰国的住处时，他正在使用阿尔法湾的管理员账号登陆。）到抓捕恐怖海盗时，法警局的效率已大幅提升。他们控制着至少两个自己的电子钱包，用于存储丝绸之路的货币，接收其他部门缴获的比特币。曾长期担任纽约南区联邦检察官办公室资产没收部主任，目前为威凯律师事务所（WilmerHale）合伙人的莎朗·科恩·莱文说：“这是一项前沿技术，我们从未这样做过。”缴获数字货币之后，法警局回归常规程序，将它与可卡因走私贩的快艇以同样方式处置：拍卖。这是一项挑战巨大的任务，因为缴获的货币数值巨大，大约有17.5万枚，占当时所有流通比特币的2%。根据熟悉案件的检察官所言，法警局采取了交错拍卖的方式，以防比特币价值暴跌。2014年6月至2015年11月期间举行的四次拍卖中，法警局以每枚均价379美元售出了丝绸之路比特币。比特币价在此后一路飙升。今年1月的另一场拍卖中，法警局售出3,813枚比特币，净赚4,500万美元，每枚比特币的价值高达约1,1800美元。按照这样的价格，法警局出售囤积的丝绸之路数字货币本可赚取21亿美元，足以支付法警局的年度预算，但在2014和2015年，它只净赚了6600万美元。与此同时，身价亿万的风险投资家蒂姆·德雷珀以每枚约600美元的价格买下了3万枚丝绸之路网站的比特币，这或许是他这十年来最佳的投资。德雷珀对《财富》杂志说，拍卖过程“十分顺利”，而且他一枚也没卖出。他补充道：“为什么我要用过去交换未来呢？”