A warning that unidentified hackers broke into an agency of the US federal government and stole its data is troubling enough. But it becomes all the more disturbing when those unidentified intruders are identified—and appear likely to be part of a notorious team of cyberspies working in the service of Russia’s military intelligence agency, the GRU.

Last week the Cybersecurity and Infrastructure Security Agency published an advisory that hackers had penetrated a US federal agency. It identified neither the attackers nor the agency, but did detail the hackers’ methods and their use of a new and unique form of malware in an operation that successfully stole target data. Now, clues uncovered by a researcher at cybersecurity firm Dragos and an FBI notification to hacking victims obtained by WIRED in July suggest a likely answer to the mystery of who was behind the intrusion: They appear to be Fancy Bear, a team of hackers working for Russia’s GRU. Also known as APT28, the group has been responsible for everything from hack-and-leak operations targeting the 2016 US presidential election to a broad campaign of attempted intrusions targeting political parties, consultancies, and campaigns this year.

The clues pointing to APT28 are based in part on a notification the FBI sent to targets of a hacking campaign in May of this year, which WIRED obtained. The notification warned that APT28 was broadly targeting US networks, including government agencies and educational institutions, and listed several IP addresses they were using in their operations. Dragos researcher Joe Slowik noticed that one IP address identifying a server in Hungary used in that APT28 campaign matched an IP address listed in the CISA advisory. That would suggest that APT28 used the same Hungarian server in the intrusion described by CISA—and that at least one of the attempted intrusions described by the FBI was successful.

“Based on the infrastructure overlap, the series of behaviors associated with the event, and the general timing and targeting of the US government, this seems to be something very similar to—if not a part of—the campaign linked to APT28 earlier this year,” says Slowik, the former head of Los Alamos National Labs’ Computer Emergency Response Team.

Aside from that FBI notification, Slowik also found a second infrastructure connection. A report last year from the Department of Energy warned that APT28 had probed a US government organization’s network from a server in Latvia, listing that server’s IP address. And that Latvian IP address, too, reappeared in the hacking operation described in the CISA advisory. Together, those matching IPs create a web of shared infrastructure that ties the operations together. “There are one-to-one overlaps in the two cases,” Slowik says.

Confusingly, some of the IP addresses listed in the FBI, DOE, and CISA documents also seem to overlap with known cybercriminal operations, Slowik notes, such as Russian fraud forums and servers used by banking trojans. But he suggests that means Russia’s state-sponsored hackers are most likely reusing cybercriminal infrastructure, perhaps to create deniability. WIRED reached out to CISA, as well as the FBI and DOE, but none responded to our request for comment.

Although it doesn’t name APT28, CISA’s advisory does detail step-by-step how the hackers carried out their intrusion inside an unidentified federal agency. The hackers had somehow obtained working usernames and passwords for multiple employees, which they used to gain entry onto the network. CISA admits it doesn’t know how those credentials were obtained, but the report speculates that the attackers may have used a known vulnerability in Pulse Secure VPNs that CISA says has been exploited widely across the federal government.

The intruders then used command line tools to move among the agency’s machines, before downloading a piece of custom malware. They then used that malware to access the agency’s file server and move collections of files to machines the hackers controlled, compressing them into .zip files they could more easily steal.