Russia’s GRU military intelligence agency has carried out many of the most aggressive acts of hacking in history: Destructive worms, blackouts, and—closest to home for Americans—a broad hacking-and-leaking operation designed to influence the outcome of the 2016 US presidential election. Now it appears the GRU has been hitting US networks again, in a series of previously unreported intrusions that targeted organizations ranging from government agencies to critical infrastructure.

From December 2018 until at least May of this year, the GRU hacker group known as APT28 or Fancy Bear carried out a broad hacking campaign against US targets, according to an FBI notification sent to victims of the breaches in May and obtained by WIRED. According to the FBI, the GRU hackers primarily attempted to break into victims’ mail servers, Microsoft Office 365 and email accounts, and VPN servers. The targets included “a wide range of US based organizations, state and federal government agencies, and educational institutions,” the FBI notification states. And technical breadcrumbs included in that notice reveal that APT28 hackers have targeted the US energy sector, too, apparently as part of the same effort.

The revelation of a potentially ongoing US-targeted GRU hacking spree is especially troubling in light of the GRU’s past operations, which have often gone beyond mere espionage to include embarrassing email leaks or even disruptive cyberattacks. APT28 hackers in particular have been the subject of US indictments alleging hack-and-leak operations targeting both the 2016 US election and the Worldwide Anti-doping Agency. The latter attack was in apparent retaliation for the International Olympic Committee banning Russia from the 2018 Olympics for performance-enhancing drug use.

“Although not all motives are clear, we can make judgments based on the nature of the target as seen through past indictments,” an FBI spokesperson wrote in a statement responding to WIRED’s request for further comment on the notification sent to APT28 hacking victims. The FBI also says that the GRU hacking campaign has likely continued into recent months. “An Advanced Persistent Threat is just that,” the spokesperson added, referring to the APT acronym from which APT28 takes its name. “There is an expectation of continued activity.”

According to the FBI’s victim notification, the APT28 hackers have gained access to networks via spear-phishing emails sent to both personal and work email accounts. They’ve also used password-spraying attacks, in which hackers try common passwords across many accounts, as well as brute force attacks that guess a long list of passwords against one or a small number of accounts.

Within days of the FBI’s notification being sent to victims in early May, the NSA issued a public advisory that Sandworm, a separate but closely linked GRU hacker group, was exploiting a vulnerability in Exim mail servers to target victims. The FBI told WIRED it knew of no connection between that Exim exploitation and the APT28 campaign.

One staff member at an affected organization told WIRED that the IT staff had seen no sign of a successful phishing attack—but nonetheless found that the hackers had accessed their email server. “Once they were on the server they stole entire mailboxes,” says the staffer, who asked that WIRED not reveal either their identity or the organization they work for.

The organization was eventually notified by the FBI that they had in fact been breached by APT28. “The natural worry is, am I the next John Podesta?” the staffer says, referring to the Hillary Clinton campaign director whose emails were stolen and leaked by APT28 ahead of the 2016 election. “Reading the victim notification and realizing how many different organizations were probably targeted, it just underscores that exactly what we worried about in 2016 is something that Russia is literally still doing as we speak.”