A notice to people using Bitcoin for illicit purposes: you can run, but it’s getting a lot harder to hide. Law enforcement officials are using Bitcoin’s public ledger, called the blockchain, to follow the digital money and track down suspected criminals using it.

As the most popular cryptocurrency, Bitcoin has helped fuel the rise of ransomware attacks—extortion schemes, like the recent WannaCry cyberattack, in which hackers hold the contents of a victim’s computer hostage until they get paid. Criminals can use Bitcoin to collect ransoms easily and without having to reveal their identities. The currency has also been associated with online drug sales, money laundering, and sex trafficking.

But while Bitcoin users can withhold their identities, they can’t avoid revealing other information that can be useful to investigators. Every Bitcoin transaction is recorded on its blockchain, a publicly accessible record of all transactions made using the currency. Blockchains “provide a really useful source of truth,” says Jonathan Levin, cofounder of Chainalysis, which develops software tools for analyzing blockchain data. Its products can help investigators draw inferences about how people are using the currency.

Chainalysis combines its analysis with other publicly available information to identify users through the unique strings of numbers they use on the blockchain, called addresses, and then map how they move funds around. This technique can be used to do things like identify the Bitcoin exchanges where the users of a gambling site are converting their bitcoins into dollars (see “Mapping the Bitcoin Economy Could Reveal Users’ Identities”).

Chainalysis’s tools are clearly valuable to criminal investigators. Since 2015, the company has supported investigations by the U.S. Internal Revenue Service, the Federal Bureau of Investigation, the Securities and Exchange Commission, the Drug Enforcement Administration, Immigration and Customs Enforcement, and Europol. In most cases, says Levin, investigators turn to Chainalysis when they already have some kind of lead, like a Bitcoin address they found among a suspect’s possessions. If they can determine that a suspect is using a particular exchange, they can use a court order get more information from that exchange.